For most felonies, grand jury hearings are swift, bare-bones proceedings. Prosecutors present enough evidence to show it is probable that the defendant, who rarely testifies, committed a crime, and ask the jury to vote for an indictment. Several cases are usually processed in a single day.

But because most prosecutors impanel a special grand jury to investigate police-related deaths, they insulate themselves from the final decision, while appearing to fulfill the public desire for an independent review, legal experts said. The inquiries often go on for weeks or months, with testimony from several witnesses.

The proceeding is transformed into a trial of sorts, behind closed doors but without cross-examination. Prosecutors control what witnesses appear and in what order, legal scholars said.

In most cases, the officer provides his or her account; prosecutors can decide to let an officer’s version of events go unchallenged or to discredit it with cross-examination. They can do the same with other witnesses.

“If the prosecutor wants an indictment she or he is probably going to get one because they do have so much control over the grand jury,” said Andrew D. Leipold, a law professor at the University of Illinois who is an expert on grand juries. “The accountability for the decision to charge or not to charge rests with the prosecutor, not with the grand jury.”

The grand jury investigating the death of Eric Garner on Staten Island sat for nine weeks and heard 50 witnesses, including Officer Daniel Pantaleo, who was videotaped as he used his arm to choke Mr. Garner from behind during a fight to subdue him. A medical examiner ruled Mr. Garner died because of the compression of his chest and neck during the struggle, but also listed his obesity, asthma and high blood pressure as contributing factors. Mr. Garner said several times that he could not breathe.

Geoffrey P. Alpert, a criminologist at the University of South Carolina who studies the use of force, said police officers are rarely indicted when they express remorse to jurors, admit they made a mistake, and stress that they were following their training, as Officer Pantaleo had. In shooting cases, officers often testify that they perceived a deadly threat and acted in self-defense. This stance can inoculate them even if the threat later turns out to be false.