Charleston County Sheriff's Office, Associated Press
This photo provided by the Charleston County, S.C., Sheriff's Office shows Patrolman Michael Thomas Slager on Tuesday, April 7, 2015. Slager has been charged with murder in the shooting death of a black motorist after a traffic stop. North Charleston Mayor Keith Summey told a news conference that city Slager was arrested and charged Tuesday after law enforcement officials saw a video of the shooting following a Saturday traffic stop.

NORTH CHARLESTON, S.C. — A young man whose cellphone video put a South Carolina police officer in jail on a murder charge said Wednesday that he gave the recording to the dead man's family because if it was his relative who was killed, he "would have liked to know the truth."

Feidin Santana told NBC News that while walking to work, he noticed Officer Michael Thomas Slager controlling Walter Lamer Scott on the ground, and began recording when he heard the sound of a Taser. "Mr. Scott was trying just to get away from the Taser," Santana said.

Slager initially claimed he fired in self-defense after the suspect he had pulled over for a broken brake light grabbed his Taser.

Santana's recording documented a different scenario. It begins at a moment when both men standing, as Scott pulls away from the officer and an object appearing to be a stun gun falls to the ground, trailing wires. As the unarmed man runs away, Slager then pulls out his Glock pistol and fires eight times at the back of the 50-year-old man, until he crumples to the ground about 30 feet away.

After the video was made public Tuesday by a lawyer for the dead man's family, Slager, 33, was swiftly charged with murder and fired, and a judge ordered him jailed without bond on the charge that could carry a sentence of 30 years to life in prison.

But that did little to quell the outrage of an angry crowd at North Charleston's City Hall, where the mayor and police chief were shouted down with calls of "no justice, no peace."

Not once in the moments recorded by Santana can the officer be heard yelling "stop" or telling the man to surrender. Moments after handcuffing the dying man face-down on the ground, Slager walks back to pick up what appears to be the Taser, then return and drop it at Scott's feet as another officer arrives to check the dying man's condition. Then he picks it up again after exchanging words with the other officer.

The video changed everything, authorities and advocates said Wednesday.

"What if there was no video? What if there was no witness, or 'hero' as I call him, to come forward?" the Scott family's lawyer, L. Chris Stewart, told The Associated Press. "We didn't know he existed. He came out the blue."

Mayor Keith Summey announced that he's ordering 150 more body cameras so that every uniformed officer on the street will wear one, a key demand of the Black Lives Matter movement that is growing nationwide. For his part, Police Chief Eddie Driggers said "I have watched the video. And I was sickened by what I saw. And I have not watched it since."

Their news conference was meant to quiet the uproar, but both were interrupted with shouted questions they said they could not answer, since the investigation was turned over to state law enforcement.

The video also prompted the FBI and the Justice Department's civil rights prosecutors to announce a federal probe Wednesday. At the White House, spokesman Josh Earnest said the video is "awfully hard to watch" and said he wouldn't be surprised if President Barack Obama has seen it, "given the amount of media attention that this issue has received."

Outside City Hall, local organizer Muhiydin D'Baha repeatedly hollered, "Eight shots in the back!" through a bullhorn. The crowd yelled, "In the back!" in response, aiming to coin a new phrase to supplant the "hands up, don't shoot!" refrain that grew out of other officer-involved killings.

But Stewart appealed to keep protests peaceful, saying any violence will undo the family's success so far in making the system work. He also plans to sue the police department, which he accused of acting decisively only because the video exposed what really happened.

Investigators also are reviewing a police dash-cam video that may show the beginnings of the traffic stop, police said, and audio of the police radio traffic revealed the sounds of Slager breathing heavily as he chases Scott into the empty lot and calls for backup, among other things. A passenger in Scott's car also was detained, according to the police reports.

The black police officer who appears in the video checking the dying man's condition, Clarence Habersham, made no mention of Slager or any of his actions in his brief official incident report, according to a copy obtained by the AP.

Months or years have passed in many other police-involved shootings before accused officers are fired, and few face criminal charges. Nationwide protests intensified after grand juries declined to indict police in the killings of Michael Brown in Ferguson, Missouri, and Eric Garner on Staten Island, New York.

But the swift action in this case did not satisfy Lance Braye, a 23-year-old demonstrator who hopes the video will help people understand that even officers can lie to save themselves when they do wrong.

"All you have to do is look at the story that was told before the video came out," Braye said.

Scott had four children, was engaged and had been honorably discharged from the U.S. Coast Guard, with no violent offenses on his record, Stewart said. He worked at a trucking distribution center, and may have feared being jailed for being behind on child support payments, his family said.

Slager also was a Coast Guard veteran, and his wife is eight months pregnant, the mayor said, which is why the city will keep paying the family's health insurance premiums despite his firing.

Santana, who has been cooperating with investigators, acknowledged that "the officer also has his family."

"But I think that he made a bad decision. And you pay for your decisions in this life, I think," Santana said.

Smith reported from Charleston, South Carolina.

Associated Press writers Tom Foreman Jr. in Charlotte, North Carolina; Michael Biesecker in Raleigh, North Carolina; and Eric Tucker in Washington.