The absence of any showing that, before searching the vehicle, the police had probable cause to believe that there was contraband, an illegally carried weapon, in the cab distinguishes this case from cases on which the Commonwealth relies. See Commonwealth v. Ortiz, 376 Mass. 349, 353-355 (1978); Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 476-479 (1981) (illegal possession of a loaded shotgun in a motor vehicle); Commonwealth v. Concepcion, 10 Mass. App. Ct. 613, 616-617 (1980); Commonwealth v. Piso, 5 Mass. App. Ct. 537, 541-542 (1977). At oral argument, the Commonwealth disclaimed reliance on any claim that the search was a lawful inventory search of the vehicle, conducted because the defendant was arrested and the cab had to be taken to a safe place and secured.

While the weapon used in the mugging was found nearby, no fingerprints tying it to the defendants rather than to the other couple were taken. Because the jury may have considered the defendants' silence as implied admissions of guilt, underscoring their identity as the assailants, we cannot conclude that the error was harmless under the circumstances. See Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 481 (1981). 2.

See Commonwealth v. Boris, 318 Mass. at 317; Commonwealth v. Burke, 339 Mass. 521, 532 (1959). No admission by silence may be inferred, however, if the statement is made after the accused has been placed under arrest, see Commonwealth v. Kenney, 12 Met. 235, 238 (1847); Commonwealth v. Morrison, 1 Mass. App. Ct. 632, 634 (1973); Commonwealth v. Cohen, 6 Mass. App. Ct. 653, 657 (1978), after the police have read him his Miranda rights, see Commonwealth v. Rembiszewski, 363 Mass. at 316, or after he has been so significantly deprived of his freedom that he is, in effect, in police custody, see Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 480 (1981), and cases cited. Some States go further than Massachusetts in restricting the use of adoptive admissions. For example, in Arizona and Pennsylvania, the prosecution is barred from introducing evidence of a defendant's silence in response to any question from a police officer or to any statement made in the presence of the police.

Probable cause may be based on the collective knowledge of police officers when they are engaged in a cooperative effort. See Commonwealth v. McDermott, 347 Mass. 246, 249 (1964); Commonwealth v. Lanoue, 356 Mass. 337, 340 (1969); Commonwealth v. Chaisson, 358 Mass. 587, 590 (1971); Commonwealth v. Gullick, 386 Mass. 278, 283 (1982); Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 478 n. 10 (1981); Commonwealth v. Wooden, 13 Mass. App. Ct. 417, 421-422 (1982); Commonwealth v. Marlborough, 21 Mass. App. Ct. 944, 945 (1985). Contrast Commonwealth v. Hawkins, 361 Mass. 384, 386-387 (1972).

Commonwealth v. Almeida, 373 Mass. at 270-273. Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 476-478 (1981). Contrast Commonwealth v. Silva, 366 Mass. 402 (1974).

See Commonwealth v. Riggins, 366 Mass. 81, 88 (1974) ("Implausible answers to police questions will, with other facts, support a finding of probable cause . . . as will peculiar behavior and evasive replies"). See also Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 478 (1981); Commonwealth v. Ceria, 13 Mass. App. Ct. 230, 234 (1982). As a consequence, everything that occurred thereafter after becomes academic: probable cause existed to justify the defendant's further restraint.

"The crime, at least in this simple two-participant type of conspiracy, under Massachusetts law is complete upon the formation of an agreement and a combination to commit, or cause to be committed, a crime or an unlawful act. Commonwealth v. Beneficial Fin. Co., 360 Mass. 188, 249-250 (1971), cert. denied, 407 U.S. 914 and sub nom. Farrell v. Massachusetts, 407 U.S. 910 (1972). Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 476 (1981). See Commonwealth v. Benjamin, 3 Mass. App. Ct. 604, 618 n. 27 (1975); Nolan, Criminal Law § 443 (1976 supp. 1981).

The crime of conspiracy is complete with the formation of the unlawful agreement. See Commonwealth v. Cerveny, 387 Mass. 280, 288 (1982); Commonwealth v. Benjamin, 3 Mass. App. Ct. 604, 618 n. 27 (1975); Commonwealth v. Cook, 10 Mass. App. Ct. 668, 670-671 (1980); Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 476 (1981). The defendant's lack of participation in the final preparatory stages and the commission of the robbery did not require a directed verdict as, even had his absence signified his disassociation from the venture, "withdrawal from a conspiracy has not been shown to have any effect upon the commission of the common law offense of conspiracy to commit a crime under Massachusetts law."

The evidence would have permitted a rational trier of fact to find that Charles and James had made a general agreement to rob the sporting goods store and had been deterred because of the close surveillance of James (wearing, perhaps as a partial disguise, a hat and dark glasses on a late afternoon in May) when he entered the sporting goods store about three quarters of an hour after Charles's exploratory visit. The judge could have inferred, from all the circumstances already outlined, that, as part of the brothers' plan to rob the store, see Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 476 (1981); Commonwealth v. Nighelli, 13 Mass. App. Ct. 590, 594-595 (1982), James had possessed the gun found on the ground near the point where James crossed the chain link fence while running away. The judge, however, did not draw this inference.

The evasive replies and lack of cooperation increased Howley's suspicions even though the license and registration check indicated the licenses appeared to be in order and the truck rented. See Commonwealth v. Riggins, 366 Mass. at 87-88; Commonwealth v. Chaisson, 358 Mass. 587, 590 (1971); Commonwealth v. Corridori, 11 Mass. App. Ct. 469, 478 (1981). Howley radioed for a backup cruiser, and someone in his department requested the Mansfield police to come.