The United States Justice Department has opened an investigation into the police killing of Alton Sterling, a 37-year-old black man whose death was filmed by bystanders.

Authorities said Sterling, who was killed early on Tuesday, had a gun as he was wrestled to the ground by two white police officers outside a Louisiana convenience store.

Mobile phone footage alleged to be of the incident has prompted hundreds of people to protest and caused outrage online.

Protests lasted late into the night on Tuesday in Louisiana's second largest city, Baton Rouge, with demonstrators chanting "black lives matter" and "hands up don't shoot".

Louisiana Governor John Bel Edwards and the US Justice Department announced on Wednesday afternoon that there would be an investigation by the department's civil rights division.

Warning: This video contains images some readers may find distressing

'Horrible tragedy'

Baton Rouge Police Chief Carl Dabadie Jr. called Sterling's death a "horrible tragedy" and said there were still questions about what happened.

Community leaders said they did not trust the police and demanded answers as to why Sterling was shot and killed outside the shop where he sold CDs.

The head of the National Association for the Advancement of Coloured People (NAACP) in Baton Rouge, Michael McCalahan, called for the police chief to be fired. 

READ MORE: Up Front - Do black lives matter in the US?

"We are going to turn the entire case over to the US Attorneys office and the FBI to conduct the investigation from this point," he said, shortly after the announcement.

"What we're going to do today is root out the one percent of bad police officers that go around becoming the judge, the jury and the executioner of innocent people. Period. But more specifically, innocent black lives," McCalahan said.

Police say they were called to the store on Tuesday night after an anonymous caller said that Sterling had threatened someone with a gun. 

Mobile phone footage

The video clip of the moment Sterling was killed, which cannot be verified by Al Jazeera, contains images some readers may find distressing.

The 48-second video shows two police officers pushing Sterling down to the ground. One officer is seen pressing his head against the ground. There are shouts of "Get on the f*****g ground!" and "If you move, I swear to f*****g God!". Then, at least five shots can be heard.

Baton Rouge police spokesman L'Jean McKneely told local media that officers had responded to an anonymous call that said there was a man in the area with a gun.

McKneely said two officers at the scene had an altercation with a man and one officer fatally shot the suspect.

He said police believed only one officer fired shots, and that the officer has been placed on administrative leave, which is standard department policy.

By dawn on Wednesday, protesters and friends had erected a makeshift memorial for Sterling on the white folding tables and fold-out chair he had used to sell homemade music compilations.

An autopsy showed that Sterling died of multiple gunshot wounds to the chest and back, according to East Baton Rouge Parish Coroner Dr William Clark. 

The shop's owner, Abdul Muflahi, told a local TV station that the first office to arrive to the scene had used a stun gun on Sterling and the second officer tackled the man. As Sterling fought to get the officer off of him, the first officer shot him "four to six times".

The store owner said Sterling did not have a gun in his hand at the time, but he saw officers remove a gun from Sterling's pocket after the shooting. 

The mother of Sterling's 15-year-old was distraught at Wednesday's news conference. 

"As a mother, I have now been forced to raise a son who is going to remember what happened to his father," said Quinyetta McMillion.

Source: Al Jazeera and agencies