The FBI and the US Justice Department have launched an investigation into the murder of a black man by a white policeman in South Carolina last Saturday.

North Charleston Mayor Keith Summey said investigators decided to charge officer Michael Slager, 33, after they viewed a video showing him shoot dead Walter Scott, 50, even as the man appeared to be unarmed and running away from him.

“If you make a bad decision… you have to live by that decision,” Summey told reporters. “We as a city want the family to know that our hearts and our thoughts are with them.”

The video, posted online by the New York Times, shows the officer firing eight shots at Scott, who then slumps facedown onto the grass.

The video was apparently recorded by a bystander and it quickly went viral. Chris Stewart, an attorney for Scott’s family, congratulated its author for helping shed light on the circumstances of the shooting.

“What if there was no video, what if there was no witness or hero as I call him to