Feidin Santana was on his way to work at a barbershop in North Charleston, S.C., on Saturday when he saw a police officer confronting a man. So Santana did something that has become common in the modern world: He took out his smartphone and started recording.

Santana was stunned by what happened next. Fear and shock crept into his voice as he captured Michael Slager, a white police officer, firing eight times at the back of Walter Scott, an unarmed black man who was running away.

And yet Santana kept recording, moving closer to the chain-link fence that separated him from the grim scene. When he watched the video later, he found himself wondering how he found the courage to keep recording.

“I asked the same question every single time,” he said in an interview Thursday with The Washington Post.

Santana’s footage spread like wildfire, turning an incident that had received scant attention outside the riverfront city into an explosive national news event. It also emphasized the acute power of video to establish evidence of police brutality, even when the officers say they have done nothing wrong.

Often the video is captured by someone like Santana, who was just passing by when Slager, 33, killed Scott, 50, claiming the older man had grabbed his stun gun. A bystander also recorded Eric Garner, a 43-year-old Staten Island man, saying, “I can’t breathe,” while a New York police officer had him in a chokehold. The video of Garner’s death prompted angry rallies, particularly after a grand jury declined to indict the officer involved.

In other cases, video is captured by stationary cameras. Surveillance video, for example, caught the Cleveland police officer who shot Tamir Rice, a 12-year-old, within seconds of leaving his patrol car.

“If you have video, it does establish a core fact to some degree,” David A. Harris, a law professor at the University of Pittsburgh, said in a telephone interview. “That’s what the video is good for.”

Video recordings have also played a major role in other cases, not all of them involving police shootings. In February, two Philadelphia police officers were charged with severely beating a man in 2013. These officers had filed a police report claiming that the man had attacked them, but surveillance video later showed this was untrue.

And this week, prosecutors in Florida filed charges against a Fort Lauderdale police officer caught on a cellphone video slapping a homeless man. Prosecutors said the officer falsely claimed that the man had reacted violently to a warning.

Bystanders who capture these videos often fear for their own safety, and that fear was evident in Santana’s remarks as he recorded the shooting in North Charleston last weekend.

“My life has changed in a matter of seconds,” Santana told MSNBC on Thursday. “My family’s afraid what’s going to happen next with me. I’m afraid, too, of what can happen. But I guess I feel that what I did is just, you know, look for justice in this case.”

Nine out of 10 American adults have cellphones, and most of those are smartphones, according to the Pew Research Center. Since the shooting in August of Michael Brown, a black 18-year-old, by a white police officer in Ferguson, Mo., a protest movement has emerged to demand that police routinely carry cameras on their uniforms as well as in their cars.

Slager’s car was equipped with a dash­cam when he pulled Scott over for a broken taillight Saturday morning. The video, released Thursday, shows Slager approaching Scott’s Mercedes-Benz and returning to his squad car with Scott’s driver’s license. Barely a minute later, Scott leaps out of his car and takes off running — concerned, his brother has said, about a warrant for back child support.

But the camera did not capture the shooting. North Charleston officials, seeking to reassure residents and maintain calm, said Tuesday that all of the city’s officers will be outfitted with body cameras. But that vow raises additional questions.

For example, when would the cameras be turned on? The Police Executive Research Forum (PERF), which studies policing, has recommended that officers wearing body cameras be required to activate them during every interaction with the public and any call for service.

“These cameras can help promote agency accountability and transparency,” PERF’s research found, according to a publication it released last year with the Justice Department’s Office of Community Oriented Policing Services.

But police unions — and officers themselves — have resisted such mandates. In Albuquerque, one of the few cities where officers already routinely wear body cameras, the Justice Department found that officers often failed to properly record what it described as episodes of excessive force.

Meanwhile, the cameras also raise issues of privacy, particularly when activated inside a private residence or during interviews with victims of rape or domestic abuse.

“They can do one good thing, which is create a detailed record,” Harris said. “But they have to be turned on to do that.”

Police have also tried to stop bystanders from recording their activities, despite the fact that such recordings are legal. In the Justice Department’s recent review of the Ferguson police force, investigators determined that officers there regularly stopped people from recording their interactions.

In Ferguson, a recording might have helped police avoid months of protests. After a lengthy review, the Justice Department found evidence that corroborated the account of Darren Wilson, the white officer who shot Brown, and federal investigators declined to pursue civil rights charges.

Meanwhile, in North Charleston, it is unclear whether Slager and a second police officer noticed Santana as he filmed them bending over Scott’s body, at one point checking his neck for a pulse. Santana said he thought at one point about erasing his video and leaving town, worried he might be in danger.

Still, Santana said he would “definitely” do it all again. And he urged others who see something to take out their phones and start recording.

“I don’t regret this,” he told Jonathan Capehart, an opinion writer for The Post, during an interview in New York. “There’s a lot of heroes and all out here. But they don’t want to step up and do the right thing.”