JERRY E. SMITH, Circuit Judge:

Following the denial of their motion to suppress the evidence of 4,500 pounds of marihuana found in their possession, defendants Santos Menendez Hernandez and Rigoberto Rosal were convicted by a jury of conspiracy to distribute and to possess with intent to distribute over 1,000 kilograms of marihuana and of possession with intent to distribute more than 1,000 kilograms of marihuana, both in violation of 21 U.S.C. § 841(a)(1). The defendants now argue that their convictions should be overturned based upon denial of their motion to suppress the evidence. Finding no error, we affirm.

I.

Following an informant's tip that a major drug conspiracy was afoot in the El Paso area, the Federal Bureau of Investigation (FBI) established surveillance on several suspected drug traffickers. Approximately two months into the investigation, agents observed certain activity that led them to suspect that the conspirators might be storing marihuana in a building on a used car lot. Two days after this suspicion arose, the agents observed a large tractor-trailer rig being driven into the car lot, where it remained backed up to the building for approximately an hour and a half. Agents then observed the defendants driving away in the truck. After a brief stop at a hotel where suspected drug conspirators were staying, the appellants drove east on Interstate 10.

After following the truck for approximately twenty-four hours and 600 miles, the FBI informed the Texas Department of Public Safety (DPS) that a truck believed to be carrying marihuana was traveling on the interstate with no license plates. DPS discovered the truck, pulled it over, and ordered the defendants out of the cab. At DPS Officer Mohon's request, the defendants produced their driver's licenses and an invoice describing the cargo; they failed to produce evidence of Texas Interstate Commerce Commission (ICC) Motor Carrier authorization.

After examining the invoice handed to him by Rosal, Mohon then walked to the back of the truck and opened an inspection port, a small door used to check the temperature of the cargo. Upon opening the inspection port, Mohon immediately could smell the odor of marihuana. Mohon then asked Rosal for the keys to the cargo door. Before the door was opened, officer Vandygriff obtained Hernandez's signature on the English language version of a consent form, and both appellants were placed in handcuffs. The search of the truck revealed approximately 98 bales of marihuana (4,540 pounds) located behind a few crates of mangoes.

At the suppression hearing, Mohon testified that the invoice, which was a bill of lading, was unusual in that, out of the hundreds of bills of lading he had encountered, he had never before seen one that indicated that the cargo was being shipped to the driver. Hernandez testified that he did not understand English, did not understand the nature and meaning of the consent form, and did not give permission for the search of the truck.

The district court found that the stop was permissible because the truck did not display any license plates. The court further found that Mohon opened the inspection port as part of his duties under the Texas Motor Carriers Act (the Act). Moreover, the court disbelieved Hernandez's claim that he did not speak or understand English adequately to comprehend the consent form. Accordingly, the court found, as an alternative holding, that the consent was valid and the search thus justified.

II.

We do not address whether appellants validly consented to the search in the instant case; we affirm the denial of the motion to suppress on the district court's first holding only; that is, we find that the Act permits a DPS officer to inspect the load of a vehicle where he reasonably believes the vehicle is not exempt from the coverage of the Act. We also find that the Act, in conferring such authority on DPS officers, does not violate the fourth amendment.

A.

Because the truck did not have license plates, Texas law enforcement authorities undeniably had the right to stop the truck. See Tex.Civ.Stat.Ann. arts. 6675a-3e § 5, 6701d § 153 (Vernon 1977). Defendants first argue that because the officers did not stop them for the purposes of enforcing the vehicle code violation but rather as a pretext to search for drugs, the stop itself should not have been permitted. However, this argument is foreclosed by United States v. Causey, 834 F.2d 1179, 1184 (5th Cir. 1987) (en banc), where we stated, "[S]o long as police do no more than they are objectively authorized and legally permitted to do, their motives in doing so are irrelevant and hence not subject to inquiry." See also United States v. Basey, 816 F.2d 980, 990 (5th Cir. 1987).

Moreover, the police officers would have the right to make a custodial arrest in response to the license plate violation. As explained in Basey, id. at 990 n. 17,

Texas law authorizes warrantless misdemeanor arrests if an officer has `probable cause to believe that the suspect has committed a crime in his presence.' Bodzin v. City of Dallas, 768 F.2d 722, 724 (5th Cir. 1985) (emphasis in original; citing Tex. Code Crim.Proc.Ann. art. 14.01(b) (Vernon 1977)); see also Tores v. State, 518 S.W.2d 378 (Tex.Crim.App. 1975) (stating that under Texas law officer may take driver into custody for any traffic offense except speeding).

Although the license plate violation does provide probable cause to arrest, the violation alone does not provide probable cause to search the trailer compartment of a tractor-trailer truck. Generally, warrantless searches are considered unreasonable in violation of the fourth amendment. Certain exceptions are made to this rule, however. In Basey, id. at 991, the court identified three types of justifiable warrantless searches of an automobile: (1) a search incident to a lawful custodial arrest of an occupant of the automobile; (2) a "vehicle frisk" as part of a Terry stop; and (3) a search under the "automobile exception" to the warrant requirement where probable cause and exigent circumstances are present.

The first two types will not ordinarily give officers the right to search a separated closed compartment area such as a trailer or trunk. We stated in Basey, "[W]hen a policeman has made a lawful custodial arrest of the occupant of an automobile, he may, as a contemporaneous incident of that arrest, search the passenger compartment of that automobile." Id. (quoting New York v. Belton, 453 U.S. 454, 460, 101 S.Ct. 2860, 2864, 69 L.Ed.2d 768 (1981)). See also Belton, 453 U.S. at 460 n. 4, 101 S.Ct. at 2864 n. 4. Similarly, "when reasonable suspicion exists to support a Terry stop, officers can properly search a passenger compartment for self-protection." Basey, 816 F.2d at 991 n. 20.

Permissible searches under the automobile exception, the third type of justifiable warrantless search of an automobile, must be based not merely upon probable cause to believe that a crime has been committed, but upon probable cause to believe that contraband or other such evidence that the government could properly seize is contained in the vehicle. As stated in United States v. Reyes, 792 F.2d 536, 538 (5th Cir.), cert. denied, 479 U.S. 855, 107 S.Ct. 191, 93 L.Ed.2d 124 (1986), "This circuit has consistently held that the automobile exception applies where there are both exigent circumstances and probable cause to believe that the vehicle in question contains property that the government may properly seize." Similarly, in United States v. De los Santos, 810 F.2d 1326, 1336-37 (5th Cir.), cert. denied, 484 U.S. 978, 108 S.Ct. 490, 98 L.Ed.2d 488 (1987), we stated, "A warrantless search of an automobile stopped by police officers who have probable cause to believe that an automobile contains contraband is permissible under the fourth amendment."

Thus, although the license plate violation provides probable cause for an arrest, it does not provide probable cause for a full vehicle search. We explained in Basey, "[A] separate search ..., unless the items examined were within the scope of the officers' justification for entering the dwelling, required probable cause or other independent legal basis because of the additional privacy interest invasion involved." 816 F.2d at 992.

B.

We conclude, however, that the search was authorized under Tex.Rev.Civ.Stat. Ann. art. 911b (Vernon 1964), which regulates motor carriers. Section 16(d) of that article begins, "Any License and Weight Inspector or other peace officer of the Department of Public Safety, shall have the power and authority to make arrests without warrant for any violation of this Act except rate violations."

Included within art. 911b § 16(j) is the statement,

Every such bill of lading, manifest, or bill of sale or load of commodities being transported for hire over the highways of this State shall be made available for inspection at any time and place upon the request of any officer or agent authorized to enforce the provisions of this Act under Section (d) hereof.

On its face this subsection entitles any DPS officer (and especially a license and weight inspector such as Mohon) at any time to inspect any load of commodities being transported for hire over the highways of the state.

At the trial level and on appeal, neither the defendants nor the government challenges Mohon's conclusion that the invoice from Meeho Brokerage, Inc., handed to Mohon by Rosal constituted a bill of lading. In his testimony, Mohon made much of its "suspicious nature" — that it indicated that the goods were being shipped to the driver — and that this gave Mohon reason to believe that the defendants were violating section 16(j), which makes it unlawful to "use a false or fictitious ... bill of lading."

On the invoice, "J.C. PRODUCE S.A., HUNTS POINT, NEW YORK" was typed in the "SOLD TO" space, while "BY: ROBERTO ROSAL" was typed in the "SHIP TO" space. Thus, the invoice itself does not seem to support Mohon's interpretation; the invoice states that the mangoes were to be shipped by Roberto Rosal, not to him. As J.C. Produce S.A. was listed as the purchaser, one would assume, upon reading the invoice, that the mangoes were to be shipped to J.C. Produce in Hunts Point and, as stated on the invoice, that they were to be "shipped via receiver's truck."

At the hearing, Mohon did not directly state that the mere appearance of the driver's name in the "SHIP TO" space was itself unusual, but it is a fair inference from his testimony that this was what initially looked improper to him about the invoice. Even though a perusal of the invoice indicates that the mangoes were being shipped by Rosal rather than to him, Mohon's statement that generally loads of produce will be shipped to produce companies (and hence normally a produce company's name would appear in any "SHIP TO" space) is still valid.

The fact that it was necessary to insert a "BY" before the name in the "SHIP TO" space is itself an irregularity. Indeed, use of this invoice as a bill of lading is also suspect because, under the Texas Uniform Commercial Code's definition, a "bill of lading" is "issued by a person engaged in the business of transporting or forwarding goods." Tex.Bus. Com. Code Ann. § 1.201(6) (Vernon 1968). The invoice here was issued by the seller rather than by the shipper and, moreover, it did not show the address of the shipper, which is also a violation of section 16(j).

Those irregularities, coupled with Rosal's inability to produce Texas ICC Motor Carrier authorization, were sufficient to give the officers probable cause to believe that the truck's cargo differed from what was stated on the invoice. Because we examine what the law objectively authorized Mohon to do, given the information he had and not on the basis of his subjective intent, see Causey, 834 F.2d at 1184, it is irrelevant that Mohon may have misinterpreted the invoice, so long as on its face the invoice, together with the lack of ICC authorization, supported probable cause.

Also, even if Rosal never intended the invoice to be presented as a bill of lading, a motor carrier should not be able to evade the Texas requirement of making available for inspection a valid bill of lading, manifest, or bill of sale, or the load of commodities, simply by not producing any of the subject documents. Where, as here, no proper documentation is produced, and DPS officers can reasonably conclude that the motor carrier is not exempt from the Act, there is probable cause to support a search of the load of commodities because of a probable violation of the Act. Thus, the search pursuant to section 16(j) was not unreasonable under the fourth amendment.

C.

The defendants, though, suggest that they should be excluded from the reach of section 16(j), as the last paragraph of that section states,

No provision of this Act will apply to any person who is engaged in the bona fide business of buying, selling and transporting any product or commodity when such person has in good faith purchased such product or commodity and at the time of and during the transportation thereof such person has and owns title to such product or commodity.

However, because the invoice stated that the goods were sold to a New York company, and because Rosal offered no contrary indication that he was the purchaser, the logical assumption for Mohon to have made was that Rosal was the shipper and not the owner of the mangoes. As such, the above-quoted last paragraph of section 16(j) would not operate to exempt Rosal from coverage under the Act.

In addition, section 16(j)'s authorization to inspect loads of commodities presumes that such commodities are "being transported for hire." It again follows logically (from the invoice, which represents a New York company as purchaser and Rosal as shipper) that Rosal was transporting property for hire.

Therefore, Mohon had statutory authorization for the search. Where an objective assessment by an officer would suggest that the truck in question is subject to the inspection provisions of section 16(j), the officer should be permitted to act in accordance with those provisions. If all doubts were to be resolved in favor of finding the inspection provisions of section 16(j) inapplicable, any truck driver could immunize himself from regulatory monitoring by making even the barest assertion of being beyond the reach of the Act. Hence, we find that the defendants' vehicle was lawfully detained and that the officers could reasonably believe it was not exempt from the Act. Accordingly, the search was permissible.

AFFIRMED.