WASHINGTON » Nothing has done more to fuel the national debate over police tactics than the dramatic, sometimes grisly videos: A man gasping "I can’t breathe" through a police chokehold on Staten Island, New York; a 12-year-old boy shot dead in a park in Cleveland. And now, perhaps the starkest video yet, showing a South Carolina police officer shooting a fleeing man in the back.

The videos have spurred calls from statehouses to the White House for more officers to attach cameras to their uniforms. While cameras frequently exonerate officers in shootings, the recent spate of videos has raised uncomfortable questions about how much the American criminal justice system can rely on the accounts of police officers when the cameras are not rolling.

"Everyone in this business knows that cops have been given the benefit of the doubt," said Hugh F. Keefe, a Connecticut lawyer who has defended several police officers accused of misconduct. "They’re always assumed to be telling the truth, unless there’s tangible evidence otherwise."

In the South Carolina fatal shooting, the most compelling evidence, provided by a bystander with a camera phone, was shaky and at times unfocused. But the video clearly showed the officer, Michael T. Slager, firing eight times as Walter L. Scott, 50, tried to flee after a traffic stop. The officer had said that he fired amid a scuffle, when Scott seized his stun gun and the officer feared for his safety.

"Without the video, we wouldn’t know what we know," said Matthew R. Rabo, a college student who joined a demonstration on Wednesday outside City Hall in North Charleston, where the officer in the shooting now faces a murder charge. "And what we know here is really significant: It’s the difference between an officer doing his job and an officer killing a man in cold blood."

Many cities have installed cameras in their police cruisers for years, and some — an estimated 25 percent of departments that responded to a 2013 survey — require so-called body cameras.

Those numbers are dwarfed by the millions of Americans who carry camera-equipped cellphones. As cameras become ubiquitous, the digital video is likely to become a go-to source of impartial evidence in much the same way that DNA did in the 1990s.

Video evidence is not new, of course; the tape of officers beating Rodney King in 1991 helped ignite the Los Angeles riots after the officers were acquitted. When departments began installing dashboard cameras in the 1990s, many officers opposed it. But they quickly concluded that the recordings often cleared them of wrongdoing after citizen complaints.

"For the most part, unless you are behaving badly, those things are going to back you up," said David Harris, a University of Pittsburgh law professor who studies police practices.

Many officers similarly opposed efforts to videotape confessions, but that resistance has been fading in recent years. Police organizations have endorsed the practice and Attorney General Eric H. Holder Jr., recently required the FBI to start taping interviews.

But cellphone videos taken by bystanders tend to make many police officers uncomfortable, because they have no control over the setting and often are not even aware they are being filmed until after an incident. Though the courts have held that people have a constitutional right to record the police, those who do are frequently challenged by officers.

While investigating the police department in Ferguson, Mo., after a deadly police shooting last summer, the Justice Department found that officers there were enraged to discover people recording them.

As an example, a Justice Department report cited a traffic stop in which a Ferguson officer told the driver’s 16-year-old son not to videotape him. The confrontation escalated, the officer wrestled the phone away from the teenager, and everyone in the car was arrested "under disputed circumstances that could have been clarified by a video recording," the report said.

Cellphone videos have captured police officers pushing and slapping a homeless man in Florida and shooting a man who threw rocks at officers in Washington state. In February, two Pelham, N.Y., officers retired after a video contradicted their account of an arrest of a black man.

"The ability to record has gotten so prevalent that police can no longer count on their account to be the truth," Harris, the Pittsburgh professor, said.

The increase in cellphone cameras is one reason many police unions do not oppose requirements that officers carry body cameras, said Chuck Wexler, the head of the Police Executive Research Forum in Washington. "The big push for body cameras has been driven in part by the sense that citizens have their phones and can record, and it was only part of the whole story," he said.

"We are very used to being videotaped," said Lt. Mark Wood, the executive officer in the operations division of the Indianapolis Police Department, where the department is testing body cameras. "We are under the impression that we are always being videotaped because we probably are."

Data is still spotty, but an early study in Rialto, Calif., suggests that when officers carry body cameras, they are less likely to use force. Similar studies in Mesa, Ariz., and in Britain showed that citizen complaints also decreased.

North Charleston, a city of about 100,000 people, has ordered about 100 body cameras but its officers are not yet using them. Mayor R. Keith Summey said Wednesday that he had ordered 150 more "so that every officer that’s on the street in uniform will have a body camera."

Marlon E. Kimpson, a South Carolina state senator who represents North Charleston and helped push for financing for the cameras, said he hoped they would help calm tensions between residents and officers. He said he believed a body camera would have prevented Saturday’s shooting.

"I don’t believe the officer would have behaved the way he did had he been wearing a body camera," he said.

Even without the video, it is likely that other forensic evidence would have raised questions about Slager’s account. The coroner found that Scott was shot several times in the back, and forensic examiners can typically tell whether someone was shot at close range in a scuffle or from a distance.

Nevertheless, the dramatic video pushed the shooting into the national spotlight. Eddie Driggers, the North Charleston police chief, told reporters Wednesday that he was sickened by the video.

Chris Fialko, a criminal defense lawyer in Charlotte, N.C., said that while the ubiquity of video had changed the dynamic between police and citizens, jurors still view police officers as credible, even when faced with incriminating video.

Fialko said he once represented an officer in a case where a dashboard camera captured the officer slamming a man, who appeared to offer no resistance, to the ground. The officer testified in his own defense.

"Video can lie," Fialko recalled saying in his closing argument. "The cop is the one out there, hearing what the guy is saying, and smelling the guy, and seeing his sweat, and he is acting based on years of experience."

The jury, Fialko said, acquitted the officer.

Matt Apuzzo and Timothy Williams, New York Times

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