STAHL, Circuit Judge.

In July 1993, a grand jury indicted defendants-appellants Santo Ruiz and Virgilio Ruiz, two brothers, for various crimes arising out of a December 1990 fire that destroyed their variety store in Roxbury, Massachusetts. After a ten day trial, a jury convicted the defendants of maliciously destroying by fire a building used in interstate commerce ( 18 U.S.C. §(s) 844(i)), mail fraud ( 18 U.S.C. §(s) 1341), conspiracy to commit the foregoing offenses ( 18 U.S.C. §(s) 371), and use of fire to commit a federal felony ( 18 U.S.C. §(s) 844(h)(1)). On appeal, the defendants challenge the district court's denial of their motions to acquit and for new trial. They also appeal their sentences. We affirm their convictions but vacate their sentences and remand for resentencing.

I.

Sufficiency of the Evidence

We review de novo the defendants' challenge to the evidentiary sufficiency of their convictions, construing the evidence in the light most favorable to the government. See United States v. Olbres, 61 F.3d 967, 970 (1st Cir.), cert. denied, 116 S.Ct. 522 (1995). Like the district court, we "must resolve all evidentiary conflicts and credibility questions in the prosecution's favor," and, among competing plausible inferences, we "must choose the inference that best fits the prosecution's theory of guilt." Id. With this standard of review in mind, we turn to the facts of the case.

A. Facts [5] 1. Pre-Fire Events

In February 1990, defendant Santo Ruiz opened a retail business named Brothers Fashions and Multiple Services ("Brothers Fashions"), in the basement area of a three-story residential duplex. Three of Santo's brothers, Pablo, Frederico, and co-defendant Virgilio Ruiz, shared an apartment directly above the store. The duplex comprised six residential apartments occupied by a total of sixteen residents, including the building's owner.

Santo borrowed $10,000 from another brother, William Ruiz, as the start-up capital for the store; additionally, Santo incurred a debt of $4000 to William for the store's fixtures. The $14,000 debt remained unpaid in its entirety through the time of the fire.

The store sold sundry items, including clothing, shoes, blankets, cosmetics, household products, music albums and cassettes, beverages and candy. Santo obtained the store's merchandise for cash from a variety of sources located in Massachusetts, New Hampshire, New York, Florida, and California.

Virgilio Ruiz spent much time at the store and frequently assisted Santo, who did not speak English, with needed language interpretation. Although Santo was the putative owner of Brothers Fashions, the evidence suggests that Santo and Virgilio represented to others a joint ownership and responsibility for the store. For example, both Santo and Virgilio signed the lease for the space as well as the business certificate filed with the City of Boston.

In early September 1990, Santo and Virgilio negotiated for the installation of a store security alarm and jointly signed the agreement with the alarm monitoring service. The alarm, which could detect heat and motion, was designed primarily to trigger when a burglar entered the premises after hours; a sufficient amount of smoke from fire could also trigger a response. The alarm, however, would not operate unless the subscriber activated the system by entering the proper pass code. About two weeks before the fire, Santo ceased activating the alarm system.

In early November 1990, approximately nine months after Brothers Fashions opened and about six weeks before the fire, Santo and Virgilio obtained $40,000's worth of insurance coverage for the store's contents. During discussions with the insurance agent, the defendants specifically asked about the processing and payment of loss claims. Although such information was not a usual part of the agent's initial discussions about insurance coverage, in response to this inquiry, he explained that receipts and cancelled checks would be required to prove a loss. Virgilio, representing Brothers Fashions, signed an insurance finance agreement. Coverage began shortly thereafter when the brothers tendered a down payment of approximately $750. The payment schedule provided that a first installment of approximately $250 would fall due on December 20, 1990.

Santo kept in his possession most of the time the only set of keys to Brothers Fashions. Before closing the business each day, Santo would secure the store-front with three heavy metal security shutters, each fastened with two padlocks. The store's back door, which was rarely opened, measured three-feet wide by four-feet tall and could be opened only from the inside of the store. It was secured on the inside with a sliding lock and at least one wooden cross-bar laid across the door. An outer iron grate, locked from the inside with a padlock, further secured the back door.

Less than two days before the fire, Brothers Fashions was fully stocked with merchandise displayed on clothing racks, shelves, and in display cases.

2. The Fire

Around 2:00 a.m. on Sunday, December 16, 1990, a fire raged through Brothers Fashions. The close proximity of the store to a fire station resulted in near immediate response. Nonetheless, the blaze was serious enough to warrant the services of some seventy fire department personnel and over one dozen fire vehicles.

Arriving at the scene, fire-fighters encountered the metal security shutters lying in the street in front of the store; the shutters apparently had been blown off the store-front by a powerful explosion. A fire-fighter in the first group to reach the back of the building noticed the store's back door standing open with flames shooting out from the basement. The fire-fighters extinguished the fire without forcible entry of the property.

Normally, when a store stocked with clothing burns, piles of clothing in various stages of ruin remain. After the fire at Brothers Fashions was suppressed, however, a casual inspection of the damaged store revealed very little burned or charred merchandise; rather, all that was visible were a few beverage bottles, some canned goods and minimal charred foodstuffs. The dearth of charred merchandise and the complete absence of clothing remnants in the store evince that it was nearly empty when it burned.

All of the apartment tenants, except Virgilio, escaped unharmed from the burning building. None of the tenants, including Virgilio, required fire-fighter assistance to evacuate. Virgilio suffered second and third degree burns on his ankles, a severe cut above his left eye, and large bruises below his left eye and across his chest. An ambulance carried him from the scene to the hospital, where he remained for eighteen days.

3. Post-Fire Events

Subsequent investigation revealed that the fire originated in two distinct locations within the store and burned in an unusual manner. Specifically, the burn pattern indicated that an accelerant, such as gasoline, had been poured over a counter and other areas. The building's electrical and natural gas distribution systems showed no sign of having caused or contributed to the fire. Following the fire, Boston Gas Company, the natural gas provider for the building, conducted a search of its records for the period from 1984 through the time of the fire for any reports of gas leak repairs at the building. The search revealed nothing.

Sometime within the first two weeks after the fire, John Greenaway, who had been hired for carpentry work on the premises, opened up the boarded-up store so that Santo could retrieve an empty clothes rack. In January 1991, Santo and Virgilio prepared, jointly signed and submitted a claim to their insurer — via certified mail — asserting a contents loss of some $48,000. The claim detailed various items of store merchandise and fixtures, including thousands of dollars' worth of men's and women's clothing, curtains, shoes, blankets, music albums and cassettes, videos, watches, candy, household items and toys.

Contrary to the insurance agent's earlier advice and the insurance company's request, the defendants did not submit any receipts, invoices, or other records to support their claim of loss. Instead, the defendants claimed that the fire destroyed all such records and submitted a couple of photographs purporting to represent the store merchandise at the time of the fire and a cancelled check from Virgilio's bank account for approximately $15 in cigarette tax. When asked by the insurance company, Santo could not recall the names of any of his retail merchandise suppliers.

At various times before trial, Virgilio related his version of the fire events to his attending physician, a Boston Fire Department investigator, and, in a sworn deposition, representatives of the insurance company. He professed a general inability to recall clearly and gave somewhat inconsistent accounts of the fire events. To the extent Virgilio's various stories overlap, they indicate that: he awoke in the first floor apartment to the smell of smoke, went to the living room to check the gas heater and saw smoke coming through the floor; he then heard an explosion and ran to awaken his sleeping brothers (Pablo and Frederico) to warn them of the fire; he picked up the telephone to call the fire department, but the line was dead; he proceeded to run out the front hallway of the duplex and down the stairs toward the building's front exit; on his way out to the sidewalk in front of the duplex, another explosion occurred and his "fuzzy" socks caught on fire, causing the injuries to his ankles; at some point, he ran into a door — perhaps the front door of the apartment unit — causing his bruises and the severe cut above his eye.

4. The Defendants' Case

The defendants presented a joint defense, calling to the stand members of the Ruiz family: Pablo, Frederico, Santo, and Rosa (Santo's wife). Pablo and Frederico testified that, on the night of the fire, Virgilio went to bed around midnight, and they retired soon thereafter. They stated that Virgilio later woke them up because of the fire. Frederico testified that he then dressed quickly and proceeded out the front door. As he descended the front stairs, he met Virgilio, who was bleeding from his forehead and down his face. Frederico and Virgilio ran out the front of the building together. Frederico was not injured — by flame or otherwise — as he exited. Upon reaching the sidewalk in front of the building, he noticed fire coming out of the basement store. Frederico said that he then told the fire-fighters that Pablo was still inside the building.

Pablo testified that his only avenue of escape from the burning building was the apartment's back door. He stated that when he exited the building, he observed two fire-fighters, who, he says, had been attempting to pry open the apartment's back door to let him out. Both Pablo and Frederico testified that they saw fire-fighters force open the store's back door, which, they claimed, had been closed. They further claimed that, upon the opening of the back door — which measured four-feet by three-feet — fire-fighters emerged from within the store wearing full fire-fighting regalia, including oxygen tanks and masks.

Santo testified in his defense. He explained that he initially did not insure the contents of the Brothers Fashions because he wanted first to see how the business fared. He testified that only he knew the alarm pass code, and that he failed to activate the alarm for several weeks before the fire because of the recurrence of false alarms. He had deduced that rats entering the store after hours (to eat candy) triggered the alarm, and he wanted to "kill" the rodents before further engaging the alarm. He testified that he complained about the rats to both the owner and to Greenaway, who, in turn, testified that Santo had never complained to him about a rat problem.

Santo suggested that a natural gas explosion may have contributed to the fire. Specifically, he claimed that about one month before the fire, he called Boston Gas because he smelled a natural gas leak; contrary to other trial testimony, he claimed that Boston Gas personnel then came out to the store and fixed the leak. Santo further testified that there may have been paint and paint thinner in the store, as well as household detergents, that might have acted as an accelerant for the fire and would account for the peculiar burn patterns.

Santo testified that on December 15, 1990, the day before the fire, the store was fully stocked. That night, he explained, he had closed the store between 5:00 p.m. and 6:00 p.m. and secured the front entrance. He insisted that he left the back door fully secured and that he retained in his possession the only set of store keys. He then went to his home in Dorchester. The next morning, around 6:00 a.m., he received a call at home informing him of the fire. He walked to the store (his car was not operational), arriving around 8:00 a.m..

Santo further testified that, upon his arrival at the fire scene, he saw ten to fifteen people in and around the burned store, including people from Boston Gas, and Greenaway's son. He said the people were removing half-burned items from the store. He admitted at trial that he had never previously mentioned his observance of the looters, even to the insurance company. He also testified that he did not attempt to stop the looting because the merchandise taken was, in any event, ruined.

Santo admitted that, although he did not speak or understand English and had to rely on Virgilio for translation, he fully understood his actions when he signed the lease to the store, the business certificate, the alarm system contract, and the insurance claim for $48,000 in lost inventory. Santo denied removing, or asking anyone to remove, the merchandise from Brothers Fashions before the fire. He also denied lighting the fire or asking anyone to light the fire.

The defendants painted a picture of a prosperous store that earned some $600 to $1000 in retail sales per day, suggesting the absence of a motive to collect insurance proceeds. The defendants also attempted to establish the bias of various trial witnesses. For example, they produced (contradicted) evidence of the unlawful absence of smoke detectors in the duplex, suggesting that the owner had her own motive to collect insurance proceeds. The defendants also suggested that the carpenter Greenaway was biased because he had married the owner's niece in 1990 and, upon the owner's death, the owner's niece stood to inherit the building.

B. Arson and Conspiracy Counts, Analysis [34] 1. Relevant Law

To prove a violation of 18 U.S.C. §(s) 844(i), the government must establish that the defendants: (1) maliciously damaged or destroyed, or attempted to damage or destroy, (2) by fire or an explosive, (3) a building or personal property used in interstate commerce or in any activity affecting interstate commerce. The government sought to prove that Virgilio set the fire, and Santo either aided and abetted him, or reasonably could foresee that Virgilio would set the fire in furtherance of an unlawful conspiracy.

To prove aiding and abetting on the part of Santo, the government had to prove that Virgilio committed the arson, and that Santo associated himself with, and participated in, the arson "as something he wished to bring about, and sought by his actions to make it succeed." United States v. Loder, 23 F.3d 586, 590-91 (1st Cir. 1994) (internal quotation marks and citations omitted); see Nye Nissen v. United States, 336 U.S. 613, 619 (1949). Alternatively, to convict Santo of the substantive arson offense, the government had to establish that Virgilio's setting of the fire was a foreseeable act done in furtherance of their unlawful conspiracy. See Pinkerton v. United States, 328 U.S. 640, 647 (1946).

To prove the underlying conspiracy — a predicate to Santo's Pinkerton liability as well as a substantive count in the indictment — the government needed to establish the defendants' knowledge and voluntary participation in the agreement, and an overt act in furtherance of it. See United States v. Sawyer, 85 F.3d 713, 742 (1st Cir. 1996). "The agreement need not be explicit; a tacit agreement will suffice." Id. To establish voluntary participation, the government must prove both the intent to agree and the intent to effectuate the object of the conspiracy. See id. Of course, direct evidence is not required to prove either the agreement (which may be inferred from circumstances) or the defendants' participation in it. See id.; United States v. Moran, 984 F.2d 1299, 1300 (1st Cir. 1993).

2. Application

The evidence more than adequately established that someone deliberately torched Brothers Fashions with the aid of an accelerant. Further, there is little question that the store was used in an activity affecting interstate commerce both because it was a rental property, and because the merchandise purchased for resale moved in interstate commerce. See United States v. DiSanto, 86 F.3d 1238, 1247-48 (1st Cir. 1996) (holding that "rental property is per se sufficiently connected to interstate commerce" for purposes of Section(s) 844(i)) (explaining further that the jurisdictional element is met where restaurant received food supplies that travelled in interstate commerce); see also Russell v. United States, 471 U.S. 858, 862 (1985).

The more troubling question is whether or not the evidence sufficiently proved that Santo and Virgilio were the parties criminally responsible for the fire. We note that the case against the defendants is largely circumstantial. Circumstantial evidence does not represent the proposition in question, but asserts "'something else, from which the trier of fact may either (i) reasonably infer the truth of the proposition, . . . or (ii) at least reasonably infer an increase in the probability that the proposition is in fact true.'" U.S. v. Clotida, 892 F.2d 1098, 1104 (1st Cir. 1989) (quoting 1 D. Louisell C. Mueller, Federal Evidence Section(s) 94 (1977)). Although circumstantial evidence requires an inferential step in its proof, there is "no legal distinction" between circumstantial and direct evidence in the context of a Rule 29 motion. Id.; see Olbres, 61 F.3d at 971. We recognize that "the government's proof may lay entirely in circumstantial evidence," United States v. Valerio, 48 F.3d 58, 63-64 (1st Cir. 1995); we are, however, "loath to stack inference upon inference in order to uphold the jury's verdict." Id.

The government produced direct evidence that Santo had in his possession the only keys to the store on the night of the fire, that the store's back door, rarely opened, was open at the time of the fire, and that Brothers Fashions had been emptied of almost all merchandise at that time. The jury thus could rationally infer that, because only Santo had the means to access the store that evening, he opened the back door from the inside of the store sometime before the fire. Santo's exclusive access also permits the conclusion that he was involved with the removal of the store's merchandise before the fire. The jury could infer that his new and uncorroborated testimony about looters was a fabrication to account for the lost merchandise — a fabrication intended to cover up his knowledge of and involvement in its removal.

The jury could conclude that Santo lied when he testified that he discontinued activating the security alarm shortly before the fire because of rats. From this untruth, the jury could infer that Santo's unexpressed reason for failing to engage the alarm was to nullify its ability to detect both the removal of merchandise and the fire. The jury could further find that Santo lied when he stated that he left the store's back door secured on the night of the fire and that, in fact, he left it open for Virgilio to set the fire.

A jury could rationally infer that Virgilio personally set fire to Brothers Fashions. Virgilio, the only apartment tenant injured by the fire, had severe burns on his ankles, consistent with direct and prolonged exposure to flame; yet Frederico, who exited the building in the same manner and through the same area as claimed by Virgilio, testified that he was not injured in any manner and that he did not encounter any fire until he reached the sidewalk in front of the building, where he viewed the fire emanating from the store. The jury could also choose not to credit Virgilio's unembellished and confused account of the cause of the injuries to his face and body and infer that, in fact, he suffered those injuries escaping the store (perhaps out the small back door) after lighting the fire.

The jury could further find that Santo lied when testifying that: (1) he had nothing to do with the removal of the store's contents, (2) he observed looters at the store the morning after the fire, and (3) Boston Gas repaired a gas leak at the store one month before the fire. The jury could also conclude that Virgilio lied about the source of his injuries. Lies such as these legitimately support a finding of guilt. See United States v. Hadfield, 918 F.2d 987, 999 (1st Cir. 1990) (finding inference of guilt could have been bolstered by defendant's "tall tale"); United States v. Jimenez-Perez, 869 F.2d 9, 11 (1st Cir. 1989) (explaining that the jury's disbelief of defendants' story allows legitimate inference "that the fabrication was all the more proof of their guilt").

As to motive, although the government did not produce evidence that Brothers Fashions was a failing business, the defendants clearly stood to gain from the insurance proceeds. Moreover, the jury could have given due weight to the fact that the defendants did not have insurance for the first nine months of the store's operation, obtained it only six weeks before the fire, and expressed peculiar concern about claim payments and proof of loss requirements. Further, the fire occurred just four days before the due date of the first installment payment, but after the down payment had already triggered coverage.

In sum, although not overwhelming, the evidence was sufficient to prove beyond a reasonable doubt that (1) Santo and Virgilio conspired to set afire Brothers Fashions, (2) Virgilio set the fire, (3) Santo assisted Virgilio in this task by providing access to the store and by failing to engage the alarm, and (4) Santo reasonably could foresee that Virgilio would set the fire in furtherance of the conspiracy. Although the evidence does not compel a finding of guilt, it need not "exclude every reasonable hypothesis of innocence or be wholly inconsistent with every conclusion except guilt." United States v. Laboy, 909 F.2d 581, 588 (1st Cir. 1990). The jury was "free to choose among reasonable constructions of evidence," id., and the trial evidence, as a whole, permits a conclusion of guilt beyond a reasonable doubt. See id. We cannot say that the jury's verdict on the arson and conspiracy counts was irrational.

C. Mail Fraud

Little discussion of this count is necessary. To prove mail fraud, the government must establish: "(1) the defendant's knowing and willing participation in a scheme or artifice to defraud with the specific intent to defraud, and (2) the use of the mails . . . in furtherance of the scheme." Sawyer, 85 F.3d at 723. There was more than adequate evidence to prove that Santo and Virgilio intentionally agreed to collect insurance proceeds fraudulently, and together submitted, by mail, a false insurance claim for merchandise they knew had not been lost in the fire. We will not disturb the jury's verdict on this count.

II.

Motion for New Trial

The defendants alternatively challenge the district court's denial of their new trial motions, claiming that, on the weight of the evidence, their convictions constituted a miscarriage of justice. A district court's power to order a new trial is greater than its power to grant a motion for acquittal. See United States v. Rothrock, 806 F.2d 318, 318 (1st Cir. 1986). In considering a new trial motion, the court may consider both the weight of the evidence and the credibility of witnesses. See id. at 321. Where a new trial motion is based upon the weight of the evidence, the court may not order a new trial "unless it is quite clear that the jury has reached a seriously erroneous result." See id. at 322 (internal quotation marks and citations omitted). We review the district court's ruling on a new trial motion for abuse of discretion. See id.

True, the trial evidence was in conflict. The jury, however, was not compelled to credit Virgilio's account of the manner in which he was injured, or Pablo and Frederico's testimony that fire-fighters forcibly entered the store's back door. The jury could have agreed with Virgilio's argument that, given the evident powerful explosion in the store, his injuries would have been more severe, or worse, he would not have survived had he been the arsonist. The jury was warranted, however, in rejecting this theory.

The strongest competing theory of innocence concerns Santo's involvement in the arson. Put simply, the jury could have reasonably found that Santo knew nothing of the fire before its occurrence, but acted as an accessory after the fact both to protect Virgilio and to profit from fraudulently obtained insurance proceeds. Santo insisted in his testimony, however, that he had the only means of access to the store on the night of the fire and that he left the back door secured that evening. In contrast, the government produced direct, credible evidence that the back door was not closed at the time of the fire; while the jury might have found that Virgilio opened the door, or, perhaps, that the explosion blew the door open (although no witness opined as such), given Santo's staunch refusal to allow for the possibility that someone else may have had access to the store, the jury could and did plausibly find that Santo opened the door. The evidence that Santo obtained insurance coverage and disengaged the alarm shortly before the fire constitutes further proof of his prior knowledge of, and involvement in, the arson-for-profit scheme. Finally, other than generally to deny any involvement with the arson, Santo's trial testimony did nothing to support the theory that he was an unwitting dupe in Virgilio's criminal plan.

The largely circumstantial nature of the proof in this case gave rise to competing plausible inferences, some pointing to guilt and others to innocence. The jury is charged with choosing between such inferences, see Olbres, 61 F.3d at 972, and, having had the opportunity to observe Santo's trial testimony and demeanor, it saw fit to convict him on all counts. While reasonable people could have found otherwise, "a trial judge is not a thirteenth juror who may set aside a verdict merely because he would have reached a different result." Rothrock, 806 F.2d at 322. Because we cannot say the jury reached a seriously erroneous result, we find no abuse of discretion in the court's carefully considered refusal to grant Santo or Virgilio a new trial.

III.

Jury Composition

For the first time on appeal, Virgilio complains of the absence of minority jury members, suggesting an unconstitutionally disproportionate ethnic representation in the jury venire. We consider Virgilio's unadorned and perfunctory appellate arguments waived. See United States v. Zannino, 895 F.2d 1, 17 (1st Cir. 1990) (explaining that undeveloped appellate arguments are deemed waived). Moreover, not only did he fail timely to raise the issue below, his assertions on appeal are, in any event, inadequate to meet his burden on the issue. See United States v. Pion, 25 F.3d 18, 22 (1st Cir. 1994) (explaining elements of prima facie case of unconstitutional disproportionality).

IV.

Other Asserted Trial Errors

Virgilio contends that the prosecutor asked two questions of Pablo Ruiz that were calculated to inflame ethnic prejudice in the jury. Some context is in order. Pablo testified that, although he received many years of education in the Dominican Republic and earned an advanced degree in agriculture, he could not find work in the United States because he did not speak English. When later asked by the prosecutor how he could remember at trial the time he went to bed on the night of the fire (some three-and-a-half years earlier) Pablo replied, "Well, if I can remember what I studied during 23 years, I should be able to remember a part of what happened that night." After further questioning about the fire events, during which Pablo's credibility was called into question by various inconsistencies, the prosecutor rekindled the subject of Pablo's inability to find agricultural work in the United States. The following exchange ensued:

[The Prosecutor]: What about California, . . . you ever tried to get a job in agriculture in California?

[Court overrules the objection and directs the witness to answer yes or no]

[Pablo Ruiz]: No.

[The Prosecutor]: What about Texas, did you ever —

[Court sustains objection].

In its post-trial order denying the defendants motions for acquittal and new trial, the district court found that "the now disputed cross-examination of Pablo Ruiz was proper, did not constitute a racist appeal to the jury, and, when viewed in context, was not material in any event." We agree. We have warned that "courts must not tolerate prosecutors' efforts gratuitously to inject issues like race and ethnicity into criminal trials." United States v. Saccoccia, 58 F.3d 754, 774 (1st Cir. 1995), cert. denied, 116 S.Ct. 1322 (1996). To the extent the disputed questions here unnecessarily (albeit marginally) invited ethnic or cultural prejudice, we strongly disapprove. While, however, the inquiries may have been of borderline relevance and materiality, it was Pablo who initially offered the explanation that his inability to speak English precluded his employment in the field for which he was most qualified. Under the circumstances of this case, we cannot say that the prosecutor impermissibly injected into the trial a harmful appeal to any ethnic bias in the jury by asking these two questions. See id.

Virgilio also claims that the prosecutor frequently "bellowed offensively at defense testimony" and "screamed at the jury with totally inappropriate accompanying facial moues and flailing gesticulation." Virgilio did not raise any objection to the prosecutor's style or manner at trial. In its post-trial order, the district court found that, "while the prosecutor's closing argument was loudly and passionately delivered, it did not exceed the bounds of propriety." Having failed on appeal to develop his argument or explain, with any detail, how the prosecutor's conduct prejudiced the trial, Virgilio cannot prevail on this challenge.

V.

18 U.S.C. §(s) 844(h)(1): Use of Fire to Commit Federal Felony The defendants also challenge their convictions and sentences under 18 U.S.C. §(s) 844(h)(1), which provides, in relevant part: "Whoever . . . uses fire or an explosive to commit any [federal] felony . . . shall, in addition to the punishment provided for such felony, be sentenced to imprisonment for 5 years but not more than 15 years. . . ." The indictment charged Santo and Virgilio with using fire to commit mail fraud, a violation of 18 U.S.C. §(s) 1341. The defendants claim, however, that "fire" was not used "to commit" mail fraud, but rather, the "mailing of articles" was used to commit mail fraud. They contend that fire may have been used to commit the arson, "but it was not the letter, envelope or stamp, or handwriting/printing/typing used to commit mail fraud." In the past, we have implicitly assumed the legal conclusion that defendants now challenge. See, e.g., United States v. Lombardi, 5 F.3d 568, 569 (1st Cir. 1993) (involving Section(s) 844(h)(1) conviction for using fire to commit mail fraud where defendant arranged arson fraudulently to secure insurance proceeds).

Defendants' challenge calls into question the meaning of the phrase "uses fire . . . to commit [certain crimes]." We have not previously been presented with this issue. We begin our analysis with the statute's language. See Bailey v. United States, 116 S. Ct. 501, 506 (1995). Because the word "use" is not defined by statute, we "construe it in accord with its ordinary or natural meaning." Smith v. United States, 508 U.S. 223, 228 (1993); see Bailey, 116 S.Ct. at 506. The word is "variously defined as '[t]o convert to one's service,' 'to employ,' 'to avail oneself of,' and 'to carry out a purpose or action by means of.'" Bailey, 116 S.Ct. at 506 (quoting Smith, 508 U.S. at 228-29 (internal quotation marks omitted) (citing Webster's New International Dictionary of English Language 2806 (2d ed. 1949) and Black's Law Dictionary 1541 (6th ed. 1990))).

We think it plain that the defendants "used" fire to commit mail fraud within the meaning of Section(s) 844(h)(1). The defendants set fire to Brothers Fashions "to carry out" their scheme to deceive the insurance company into making payment for claimed losses. Specifically, the fire constituted "the means" by which the defendants attempted to create the appearance of a legitimate loss of insured items. While the defendants also "used" the mails in furtherance of the scheme to defraud, that does not diminish the fact that, additionally, they "employed" or "availed themselves of" fire to effect their scheme. Cf. Smith, 508 U.S. at 230 (explaining that including one method of "us[ing] a firearm" does not result in excluding others). Finally, the defendants make no argument that the placement and purpose of the word "use" in its statutory context undermine the applicability of Section(s) 844(h)(1) here. See Bailey, 116 S.Ct. at 506-07 (considering disputed language in the context of overall statutory scheme). No basis for such an argument is apparent.

In conclusion, we see no basis to overturn the defendants' convictions and attendant penalties under 18 U.S.C. §(s) 844(h)(1).

VI.

Sentencing Guideline Challenge

Defendants appeal the court's application of the federal arson guideline. See U.S. Sentencing Guidelines Manual [hereinafter "U.S.S.G."] Section(s) 2K1.4(a)(1)-(4) (1994). In the sentencing context, we review factual determinations, which must be supported by a preponderance of the evidence, for clear error. See United States v. McCarthy, 77 F.3d 522, 535 (1st Cir.), cert. denied, 117 S.Ct. 479 (1996), and cert. denied, 65 U.S.L.W. 3505 (U.S. Jan. 21, 1997) (No. 96-5017). We review de novo sentencing issues involving questions of law, including the applicability of a relevant guideline. See id. Because the sentencing court enjoys a unique vantage point and has "special competence" in assessing the "ordinariness" of a case, we afford it substantial deference in departure decisions, which we review only for abuse of discretion. Koon v. United States, 116 S.Ct. 2035, 2047 (1st Cir. 1996).

The arson guideline provides for different base offense levels depending on the circumstances of the crime. Subsection (a)(1) of the guideline authorizes the application of a base offense level of 24 if:

the offense (A) created a substantial risk of death or serious bodily injury to any person other than a participant in the offense, and that risk was created knowingly; or (B) involved the destruction or attempted destruction of a dwelling.

U.S.S.G. Section(s) 2K1.4(a)(1) (emphasis added). Subsection (a)(2) authorizes a lower base offense level, 20, if:

the offense (A) created a substantial risk of death or serious bodily injury to any person other than a participant in the offense; (B) involved the destruction or attempted destruction of a structure other than a dwelling; or (C) endangered a dwelling, or a structure other than a dwelling.

U.S.S.G. Section(s) 2K1.4(a)(2) (emphasis added).

As indicated by the underscored language in the text of the arson guideline set forth above, the higher base offense level (24) is warranted under either the first prong of Section(s) 2K1.4(a)(1) — the knowing creation of a substantial risk of death or serious bodily injury (Section(s) 2K1.4(a)(1)(A)) — or the second prong — the destruction or attempted destruction of a dwelling (Section(s) 2K1.4(a)(1)(B)). The selection between this base offense level and the lower level (20) set forth in the subsequent section, Section(s) 2K1.4(a)(2), depends upon either the defendant's mens rea (i.e., whether or not it was "knowing") in creating the requisite risk, or the type of structure involved, i.e., dwelling or non-dwelling. If the fire involved a dwelling, the selection between the base offense levels depends on whether the defendant destroyed (or attempted to destroy) it, or merely endangered it.

The defendants offered two rationales to the district court in support of an application of a base offense level of 20: (1) it is inconceivable that they would intentionally create a substantial risk of death or serious bodily injury to their family members living directly above Brothers Fashions; and (2) they did not destroy or attempt to destroy a dwelling, but rather, a store; and the upstairs residences, only minimally damaged, were merely "endangered" incidentally to the arson offense.

The government countered that a base offense level of 24 was warranted because, under the first prong of Section(s) 2K1.4(a)(1), the defendants had only to "knowingly," not "intentionally," create the risk of death or injury. Moreover, the government argued, the defendants satisfied the "knowingly" requirement because they committed the arson at a time when they knew residents were in the very building to which they set fire. The government argued further that the second prong of Section(s) 2K1.4(a)(1) warranted a level 24 because the defendants destroyed or attempted to destroy a dwelling.

The district court observed that a pivotal question revolved around the meaning of the word "knowingly." Section(s) 2K1.4(a)(1)(A). The court opined that the definition of "knowingly" was "a little bit ambiguous" in the context of the arson guideline. It first observed that, "[o]bjectively, a rational person perhaps should know that if he set the gasoline fire in the basement, people living above would be injured." The court, however, further stated that if it interpreted "knowingly" as "intentionally," it did not believe that Virgilio Ruiz intentionally put his brothers in danger; instead, the court observed, "I think subjectively he may have had the foolish, but sincere belief that this fire could be started, that everybody could be quickly evacuated or even that the dwelling where they lived would not be destroyed, just the store would be."

The court then concluded:

Well, I'll tell you what I'm going to do. I'm going to rate this as a 22. There is a preamble to the guidelines which I cannot find right [now] that says if something doesn't perfectly fit one category or another, it is permissible to interpolate.

For example, there is a law that says if somebody is between a minor and minimal participant, you can reduce the role in the offense by 3 rather than 2 or 4 and, in [another case], I [had] to study the interpolation language at the beginning of the guidelines. It seems to me this is a [case] which generally falls between the two provisions, [Section(s) 2K1.4](a)(1) and (a)(2), essentially for the reasons [stated above]. And I think the fairest and legal[ly] most appropriate thing to do in the circumstances would be to interpolate and I'm going to rate this as a 22.

On appeal, the defendants contend that the district court erroneously "interpolated" when applying the arson guideline. They argue that if the court found inapplicable Section(s) 2K1.4(a)(1), it should have affixed the lower base offense level provided for in Section(s) 2K1.4(a)(2), rather than apply an intermediate level. The government does not appeal the sentences as calculated, but nonetheless argues that the higher base offense level was clearly appropriate and that the defendants, who benefitted from the court's "interpolation" exercise, have no basis for complaint.

The district court's use of "interpolation" to affix an intermediate base offense level was an apparent attempt to invoke a paragraph (now deleted) in the Introduction to the pre-November 1989 Guidelines Manual. See, e.g., U.S.S.G. Section(s) 1A4(b) (1987). The Sentencing Commission had designated "interpolation" as a form of departure in which a sentencing court could choose an intermediate point "between two adjacent, numerically oriented guidelines rules." Id. Effective November 1, 1989, however, the Commission deleted that interpolation provision, concluding "that it is simpler to add intermediate offense level adjustments to the guidelines where interpolation is most likely to be considered." U.S.S.G. App. C, Amendment 68 (1989). The Commission also stated, however: "This amendment is not intended to preclude interpolation in other cases; where appropriate, the court will be able to achieve the same result by use of the regular departure provisions." Id. Thus, although the issue is somewhat unclear, it appears that interpolation between guideline rules is permissible where it could be properly justified under the normal departure procedures.

Assuming that the court essentially effected a downward departure when affixing a base offense level of 22, we find that because the district court failed to make specific findings on the defendants' state of mind, we are unable to resolve either the defendants' contention that the facts warranted a level 20, or the government's argument that a level 24 was appropriate. Our inability to settle the sentencing issue absent such findings requires us to remand the case for further findings and resentencing. See United States v. Valencia-Lucena, 988 F.2d 228, 234-36 (1st Cir. 1993) (remanding for resentencing where sentencing court failed to make reasonably specific findings concerning foreseeability of drug quantity); see also United States v. Olbres, 99 F.3d 28, 30, 32 (1st Cir. 1996) (remanding for clarification of sentencing court's specific willfulness findings in context of downward departure).

In United States v. DiSanto, 86 F.3d 1238, 1256 (1st Cir. 1996), we stated:

Given the structure of the arson guidelines, we conclude that Section(s) 2K1.4(a)(1)(A) requires that the district court make a specific finding that the defendant 'knowingly' created a substantial risk of death or serious bodily injury, as opposed to merely finding that defendant recklessly (or negligently) created such a risk which would more appropriately trigger application of [Section(s) 2K1.4(a)(2)] (emphasis added).

Whether or not the defendant acted "knowingly" calls for an inquiry into his subjective state of mind when he created the requisite risk. See id. at 1255 (citing United States v. Karlic, 997 F.2d 564, 568-69 (9th Cir. 1993)). In this case, the court explored the various possibilities of a "knowledge" finding, opining first that a rational person would know that setting such a fire would cause injury, then observing that Virgilio did not "intentionally" put others in danger because he "may" have had the "foolish but sincere belief" in a swift evacuation of the residents. In the end, the court did not make any specific findings on either of the defendants' state of mind but, instead, resorted to interpolation.

The court's reluctance to make the requisite findings may have been the result of the somewhat unsettled definition in this circuit of the term "knowingly" in the arson guideline. See generally, DiSanto, 86 F.3d at 1256-58. In DiSanto, we discussed at some length the possible parameters of the word "knowingly." See id. at 1256-58. We explained that "'the hypothetical knowledge continuum' is marked by 'constructive knowledge' at one end and 'actual knowledge' at the other." Id. at 1257 (quoting United States v. Spinney, 65 F.3d 231, 236-37 (1st Cir. 1995)). In between these extremes lie various "gradations," including "notice of likelihood" and "practically certain." Id. Although we expressed our inclination "to conclude that a showing of knowledge anywhere along this continuum satisfies application of Section(s) 2K1.4(a)(1)(A)," we declined to determine the exact level of knowledge required for the guideline. Id. at 1257-58 (finding that government had established the somewhat high standard of defendant's awareness that the requisite risk was "practically certain").

Had the district court here actually found that the defendants acted knowingly when they created a substantial risk of death or serious bodily injury to someone other than themselves, we likely would have upheld the finding as not clearly erroneous, even under the stringent "practically certain" test. The evidence established that the defendants set fire to a building basement in the early hours of a winter morning, knowing that the residential units above were occupied. "It is difficult to imagine a clearer illustration of the knowing creation of a substantial risk of death or serious bodily injury." United States v. Honeycutt, 8 F.3d 785, 787 (11th Cir. 1993) (finding requisite level of knowledge where defendant, having seen people inside tavern, threw a Molotov cocktail at the outside corner of the building) (using "practically certain" test). The sentencing court, however, must make such a finding in the first instance, and we will not engage in that initial exercise on appeal.

Because we remand this case for findings and resentencing, we think it necessary to clarify further the definition of "knowingly" appearing in subsection (a)(1), but not (a)(2), of the arson guideline. See U.S.S.G. Section(s) 2K1.4. The guideline's structure "clearly suggests that there must be a meaningful distinction between the two sections." DiSanto, 86 F.3d at 1256. As we noted in DiSanto, 86 F.3d at 1257, the Ninth and Eleventh Circuits have adopted the Model Penal Code's definition of "knowingly." In those circuits, "a defendant can be found to have 'knowingly' created a substantial risk of death or serious bodily injury under Section(s) 2K1.4 only if the defendant was aware that a substantial risk of death or serious bodily injury was 'practically certain' to result from the criminal act." Karlic, 997 F.2d at 569 (emphasis added); accord Honeycutt, 8 F.3d at 787; see also Model Penal Code Section(s) 202(2)(b) (1985).

We agree with our sister circuits that the Model Penal Code's definition gives due regard to the "meaningful distinction" between the pertinent guideline sections. Indeed, we have already held that a "practically certain" finding, which is akin to, but something less than, actual knowledge, satisfies the definition of "knowingly." DiSanto, 86 F.3d at 1257-58. Thus, we adopt the Model Penal Code absent a contrary guidelines definition. We acknowledge our earlier inclination to include all gradations of the knowledge continuum in this context. Id. at 1257. To the extent, however, that "constructive knowledge" requires only that the defendant either have had a "notice of likelihood" or "should have known" of a substantial risk, see id. at 1257-58 n. 30; Spinney, 65 F.3d at 236-37, we now think something more is needed. For the purposes of the arson guideline, these particular "constructive knowledge" formulations appear to establish only "recklessness" or "negligence," both insufficient to trigger the higher base offense level. See DiSanto, 86 F.3d at 1256.

On remand, the district court should articulate specific findings of each defendant's state of mind regarding the creation of the risk. In order for the higher offense level to apply, the court certainly need not find that the defendants "purposefully" or "intentionally" created a substantial risk of death or serious bodily injury. Rather, the court need find only that Virgilio and Santo actually knew that they created such risk, or were aware that the risk was practically certain. If, on the other hand, the court supportably finds that one or both of them actually believed that no substantial risk was created under the circumstances, a finding of knowledge would not be warranted. See DiSanto, 86 F.3d at 1257 n. 29 (citing Model Penal Code Section(s) 2.02(7)(1985), which provides that a defendant's actual belief in the nonexistence of a fact precludes knowledge finding).

VII.

Conclusion

For the foregoing reasons, we affirm the defendants' convictions, vacate their sentences and remand for resentencing.