Even with indictments, juries will remain reluctant to convict police officers absent evidence of malice, said Eugene O’Donnell, a former officer and prosecutor who now teaches at the John Jay College of Criminal Justice in New York. “Tremendous incompetence, the worst kind of training, disregard for people is really not enough,” he said. “You’re going to have to go beyond that because the police are different.”

Some jurors in police cases have later made a distinction between determining whether the officer should have fired and deciding — as the law instructs — whether the officer could reasonably have feared bodily harm. In the case of Officer Mearkle in Pennsylvania, the victim lay on the ground as ordered, but did not keep his hands in full view, a video of the shooting shows.

But convictions or no, Professor O’Donnell said, the intense scrutiny brought on by a wave of activism after the shooting of Mr. Brown, followed by this year’s string of indictments, has caused “seismic changes” in policing.

“You can absolutely be sure there’s an impact on the everyday work the police do,” he said. “This is reminding the police that they should only be using deadly force as a last resort.”

“It’s an imperfect way to fix the system, the criminalization of people,” he added. “In a way, we’re doing bottom-up reform instead of top-down reform. We’re finding egregious endings and working from there instead of proactively saying the police system is part of the criminal justice system and consequently is broken. The political sector hopes that the conversation will end there, at the bottom or close to the bottom.”

Cleveland recently agreed to overhaul its police department after a Department of Justice investigation found a pattern and practice of excessive force. The department now places a first aid kit in every squad car and offers first aid training. Tamir lay unaided by the police for several minutes after he was shot.

But at a news conference Tuesday, Mayor Frank Jackson and Police Chief Calvin Williams spoke not of policy changes, but of a top-to-bottom review of the Rice shooting to determine whether and how much to discipline the two officers, the 911 operator and the dispatcher. The 911 caller who reported the gun said that it might be a toy and that the suspect was probably a child, but that information was not relayed to the officers.