A Norfolk County grand jury indicted the defendant for assault with intent to rape.1 Prior to trial, the defendant moved to dismiss the indictments because the *161grand jurors had not heard any evidence that he had been involved in criminal activity. The trial judge denied the motion. After trial to a jury, the defendant was convicted. He was sentenced to a term of twenty years at the Massachusetts Correctional Institution at Concord. Execution of the sentence was stayed by a single justice of the Appeals Court. See Commonwealth v. Allen, 379 Mass. 564, 565 n.1 (1980); Commonwealth v. Levin, 7 Mass. App. Ct. 501 (1979). We allowed the defendant’s application for direct appellate review.
The sole issue is whether the trial judge should have dismissed an indictment which was unsupported by any evidence of acts of criminality by the defendant. The defendant claims that in these circumstances the denial of his motion to dismiss the indictments was erroneous. We agree. We reverse and order that the indictment on the charge of assault with intent to rape be dismissed.
We summarize the facts. Although the defendant requested copies of the grand jury minutes, the Commonwealth did not furnish them until the day of the trial. After receiving the minutes, the defendant immediately moved to dismiss the indictments, because the grand jury heard no evidence that he had engaged in criminal activity.2
The minutes revealed that during a party to which the victim, his sister, the defendant, and a friend had been invited, one William Maloney3 tried to rape the victim. The grand jury minutes further revealed that after the victim escaped from Maloney, the victim’s sister entered the house in which the party was being held and recognized the defendant among those present.
In arguing that the indictment should be dismissed, the defendant does not dispute that generally a “court will not *162inquire into the competency or sufficiency of the evidence before the grand jury.” Commonwealth v. Robinson, 373 Mass. 591, 592 (1977), quoting Commonwealth v. Galvin, 323 Mass. 205, 211-212 (1948). See Commonwealth v. Lincoln, 368 Mass. 281, 285 (1975); Commonwealth v. Hare, 361 Mass. 262, 269 (1972). Nor does the defendant dispute that an indictment may be based solely on hearsay. Commonwealth v. Gibson, 368 Mass. 518, 522-525 (1975).4 Commonwealth v. Walsh, 255 Mass. 317, 318-319 (1926). Mass. R. Crim. P. 4 (c), 378 Mass. 849 (1979). See Costello v. United States, 350 U.S. 359 (1959). Instead the defendant claims that unless the grand jury has heard sufficient evidence (whether competent or hearsay) to establish the level of probable cause required to support an arrest or search warrant, an indictment must be dismissed.5 The de*163fendant argues that since the grand jury heard no “reasonably trustworthy information . . . sufficient to warrant a prudent man in believing that the defendant had committed or was committing an offense,” Commonwealth v. Stevens, 362 Mass. 24, 26 (1972), quoting Beck v. Ohio, 379 U.S. 89, 91 (1964), the indictment against him is “fatally defective.” Connor v. Commonwealth, 363 Mass. 572, 574 (1973). Thus, he asserts that “all subsequent proceedings taken in reliance upon the indictment were void.” Id. We agree.
We see no material difference between this case and Con-nor v. Commonwealth, supra. In Connor, the absence of any evidence identifying the defendant as the person whom the grand jury intended to indict was sufficient to justify the dismissal of the indictment. We believe that the grand jury’s failure to hear any evidence of criminal activity by the defendant justifies dismissal of this indictment. We, therefore, hold that at the very least the grand jury must hear sufficient evidence to establish the identity of the accused, Connor v. Commonwealth, supra, and probable cause to arrest him, Lataille v. District Court of E. Hampden, 366 Mass. 525, 531 (1974). See note 4, supra. A grand jury finding of probable cause is necessary if indictments are to fulfil their traditional function as an effective protection “against unfounded criminal prosecutions.”6 Lataille v. District Court of E. Hampden, supra at 532.
The judge recognized that the grand jury did not have before it any evidence of criminality by the defendant. However, he ruled that the grand jury did hear sufficient evidence to indict the defendant as Maloney’s accomplice. But “mere presence at the commission of the wrongful act *164and even failure to take affirmative steps to prevent it do not render a person liable as a participant.” Commonwealth v. Benders, 361 Mass. 704, 708 (1972). See Commonwealth v. Morrow, 363 Mass. 601, 609 (1973); Commonwealth v. Knapp, 9 Pick. 495, 517 (1830). Only “one who aids, commands, counsels or encourages the commission of a crime while sharing with the principal the mental state required for the crime is guilty as a principal.” Commonwealth v. Soares, 377 Mass. 461, 470 (1979).
In this case, the grand jury had no evidence before it that the defendant commanded, counseled, or encouraged Maloney to assault the victim or even that he was present at the assault. Since the indictment was not supported by evidence that the defendant was Maloney’s accomplice, the defendant’s motion to dismiss should have been allowed.7
The judgment of the Superior Court is reversed, the verdict is set aside, and the case is remanded to the Superior Court where the indictment is to be dismissed.8
So ordered.