North Charleston police officer Michael Slager is seen in an undated photo released by the Charleston County Sheriff's Office in Charleston Heights, South Carolina.[Photo/Agencies]

CHARLESTON, S.C.?- A white South Carolina police officer was charged with murder on Tuesday after a video showed him shooting eight times at the back of a 50-year-old black man who was running away.

North Charleston Mayor Keith Summey said state investigators decided to charge officer Michael Slager, 33, with the murder of Walter Scott after they viewed the video of the incident, which followed a traffic stop on Saturday morning.

The FBI and US Justice Department have begun a separate investigation.

"When you're wrong, you're wrong," Summey told reporters. "If you make a bad decision, I don't care if you're behind the shield or just a citizen on the street, you have to live by that decision."

The incident began after Scott was pulled over for a broken brake light, police said.

A video of the encounter published by the New York Times shows a brief scuffle between Slager and Scott before the latter begins running away.

The video, apparently recorded by a bystander, shows the officer firing eight shots at Scott as he runs away. Scott then slumps facedown onto the grass.

A police incident report says that Slager, who joined the department in 2009, told other officers Scott had taken his stun gun. In the video, Scott does not appear to be armed while fleeing from Slager.

With the victim lying facedown on the ground, Slager approaches him and puts him in handcuffs, the video shows. The officer then walks several paces back to where he opened fire, before returning to Scott and appearing to drop an object next to him on the ground, it shows.

Chris Stewart, an attorney for Scott's family, did not immediately respond to messages seeking comment.

The incident comes at a time of heightened tension over the deadly use of force by US police, particularly by white police officers against black men - including 18-year-old Michael Brown, who was shot and killed by a white police officer last year in Ferguson, Missouri, sparking nationwide protests.

Social media sites such as Twitter were a frenzy of reaction, mostly by people commenting that without the video, no action might have been taken against the police officer.

"I guarantee if there was no video, the evidence would have automatically matched cops versions," one person tweeted.

Added another: "Imagine how many times throughout history they got away with murder because there wasn't a camera."

North Charleston is a community of about 100,000 residents, nearly half of whom are black, according to 2010 US Census data. It is far more diverse than South Carolina at large, where blacks made up just 28 percent of the 2010 population.

The federal probe will be handled by the US Justice Department's Civil Rights Division and the South Carolina US Attorney's Office together with the FBI, the Justice Department said.