Mr. Baldwin has maintained that he did not pull the trigger. Instead, he told investigators, he pulled back the hammer on the gun — and when he let go, the gun went off.

But the forensic gun expert the prosecutors hired to conduct a more detailed analysis found that the actor would have had to put pressure on the trigger for the gun to have gone off, providing the basis for Tuesday’s announcement.

The report by the forensic analyst, Lucien C. Haag, did not directly address whether the gun, a Pietta replica of a 1873 revolver, had been modified. But it said that it required about two pounds of pressure on the trigger to discharge — a small amount when compared to typical modern weapons but not unusual for a single-action revolver of that kind, experts said.

“Although Alec Baldwin repeatedly denies pulling the trigger, given the tests, findings and observations reported here, the trigger had to be pulled or depressed sufficiently to release the fully cocked or retracted hammer of the evidence revolver,” Mr. Haag wrote in the report, which was released through a public records request. He also included photos of Mr. Baldwin handling a revolver on set earlier in filming, highlighting the actor’s finger on or near the trigger as he was cocking the gun.

Before Mr. Haag tested the gun, it had been examined by the F.B.I., which broke parts of it during testing, a complication that is certain to be raised by Mr. Baldwin’s defense. In order to test the gun, Mr. Haag replaced the damaged parts with new ones from the same type of gun.

In making their case before the grand jury, the prosecutors will have to contend with the fact that there was not supposed to be any live ammunition on the film set, and that both the film’s armorer and first assistant director had checked the gun before handing it to Mr. Baldwin. Ms. Morrissey said that her team believes a grand jury should make the decision “about the level of culpability of the person holding the gun in their hand.”