After deadlocked juries led to two mistrials in the case of Raymond M. Tensing, a former Cincinnati police officer who shot and killed an unarmed black motorist, prosecutors said Tuesday that they were preparing to drop charges against him.

“It’s my decision that he will not be retried a third time,” said Joseph T. Deters, the head prosecutor in Hamilton County, speaking at a news conference at his office in downtown Cincinnati. “After talking to these jurors, there is not a likelihood of success at trial,” he said.

Mr. Deters said that federal prosecutors in Ohio had called his office about Mr. Tensing’s case, and that the matter has now been referred to them. The office of the United States attorney for the Southern District of Ohio, Benjamin C. Glassman, said in a statement that it was reviewing evidence from the trials to see if there were “possible federal civil rights offenses warranting investigation and potential prosecution.”

The decision not to again retry Mr. Tensing, who killed Samuel DuBose with a shot to the head during a traffic stop in 2015, is a setback for activists seeking greater accountability in police shooting cases. It comes amid a series of acquittals of police officers who, like Mr. Tensing, who were captured on video firing fatal shots at black men.

“Tomorrow will be two years” since the death of Mr. DuBose, his sister Terina Allen said after Mr. Deters’s announcement. “And they’re just going to let this man walk free in the state of Ohio?”

“I don’t care if you’re white or black,” she said. “This fight, to me, is about law enforcement having a free rein to shoot and kill people.”

Ms. Allen recited a grim list of black people who have been killed in the last few years by police officers, including Philando Castile in Minnesota, and Tamir Rice, a 12-year-old fatally shot in Cleveland. Their deaths, along with Mr. DuBose’s, became part of a painful national debate over race and policing.

Mr. Deters charged Mr. Tensing with murder and voluntary manslaughter 10 days after the shooting on July 19, 2015, and released body camera footage that showed a routine traffic stop devolve into violence in a matter of seconds.

In the footage, Mr. Tensing is seen pulling over a green Honda Accord that Mr. DuBose was driving because, the officer says, of a missing front license plate. Despite repeated requests from Mr. Tensing, Mr. DuBose does not produce a drivers’ license. Mr. Tensing asks Mr. DuBose to remove his seatbelt, and places his hand on the car door, to open it. Then, as the footage turns shaky, Mr. DuBose closes the car door with one hand and starts the car with his other hand. Mr. Tensing reaches into the car and yells “Stop!” twice, the video shows, at which point the officer fires his gun once, striking Mr. DuBose in the head.

“This office has probably reviewed upwards of 100 police shootings and this is the first time that we thought, ‘This is without question a murder,’” Mr. Deters said, announcing charges in 2015 and describing the shooting as “asinine” and “senseless.”

Since then, though, the case has been anything but straightforward.

Last fall, Mr. Tensing’s first trial ended with a mistrial. After Mr. Deters said his office would prosecute the case a second time, more complications emerged. Halfway through the second trial, prosecutors tried to add a lesser charge of reckless homicide, but Judge Leslie Ghiz concluded that it was, in essence, too late. The second trial ended last month, with a jury that said it was almost evenly split.

Mr. Tensing, who was 25 at the time of the shooting and was fired from the University of Cincinnati after he was charged, has taken the stand in both of his trials. He told jurors that he fired his gun because he believed his arm was stuck in Mr. DuBose’s steering wheel and that he was in danger of being dragged by the car.

“I felt my body moving with this car, falling backward, and instinctively I just reached for my gun,” Mr. Tensing said, during his second trial, adding that he shot “to stop the threat.”

Prosecutors used video analysis to tell the jury Mr. Tensing was neither threatened nor stuck in the steering wheel, but Mr. Tensing’s lawyers urged jurors to consider Mr. Tensing’s perceptions, not the “20/20 hindsight” from the video.

Mr. Tensing’s second trial, which ended late last month, was the third time in a matter of days that a jury opted not to convict a police officer charged in the death of a black man, even after seeing video of the fatal shootings. A jury in St. Paul acquitted Officer Jeronimo Yanez, who shot and killed Philando Castile, a cafeteria worker, while he was in a car with his fiancée and her daughter. Jurors in Milwaukee acquitted Officer Dominique Heaggan-Brown after watching video of him shooting a fleeing suspect, Sylville K. Smith.

Mr. Deters has been under pressure from supporters of Mr. DuBose’s family and of Mr. Tensing in the weeks since a mistrial was declared. An online petition urging Mr. Deters to drop the charges had more than 5,500 signatures on Tuesday morning. And on Friday, supporters of Mr. DuBose called for another trial outside of Mr. Deters’ office.

“This case is not done,” Donyetta D. Bailey, president of the Black Lawyers Association of Cincinnati, said. “The DuBose family is not done.”