A video has emerged appearing to show two white police officers holding down and shooting dead a black man during an incident in the US state of Louisiana.
The confrontation took place in the state capital, Baton Rouge, on Tuesday after reports of a man threatening people with a gun outside a shop.
A post-mortem examination showed the victim, Alton Sterling, 37, died of gunshot wounds to the chest and back.
The video shows two officers wrestling a man in a red shirt to the floor.
Family members and local leaders have called for a federal investigation, adding that they do not trust local law enforcement officials,
Mr Sterling's family told reporters they want to know why Mr Sterling was killed.
In the video, the man appears struggles and one of the officers pins his arm to the floor with his knee and then appears to pull out his gun and point it at the man.
A voice is heard shouting: "He's got a gun. Gun." Then shots ring out and the camera moves away.
A woman - apparently a bystander - can be heard saying "Oh my God", followed by another asking: "They shot him?" The first woman replies: "Yes."
The incident comes amid heightened tension in the US over the deaths of African-American men at the hands of police.
There are more than 1,000 deadly shootings by police in the US each year, and disproportionate numbers of those killed are black Americans.
Hours after the shooting, a crowd of about 200 people protested at the scene, blocking roads and chanting: "Black lives matter".
They were moved on by police but organisers said they would reassemble outside City Hall later.
Police spokesman Cpl L'Jean McKneely said the officers had been called in after reports that a man selling CDs outside a convenience store had threatened someone with a gun.
Mr Sterling, a father of five, died at the scene.
Cpl McKneely said the two officers involved had been placed on administrative leave, which is standard procedure.
The short mobile phone footage was circulated on social media and on local news outlets hours after the shooting.
Sandra Sterling, an aunt who said she raised Mr Sterling after his mother died, told the Washington Post the video was difficult to watch.
"That video is everywhere now," she said.
"It hurts me to see it. I can see the picture but I don't want to hear the sound. The sound gets me. It gives me an anxiety attack when I hear the sound."
On Wednesday, Michael McClanahan, president of Baton Rouge's NAACP chapter, called on the city's police chief to resign after Mr Sterling's death.
Cameron Sterling, Mr Sterling's 15-year-old son, broke into tears at a news conference as his mother spoke.
"He had to watch this as this was put all over the outlets," Cameron Sterling's mother told reporters. "As a mother I have now been forced to raise a son who is going to remember what happened to his father."
Mr Sterling's wife added her husband "simply tried to earn a living to take care of his children".
Abdul Muflahi, who owns the shop outside of which the incident took place, told WAFB TV station that Mr Sterling was shot "four to six times".
He said the victim did not appear to have a gun in his hand during the altercation although he saw officers remove a gun from the man's pocket following the shooting. Police could not confirm his account.
Denise Marcelle, a member of the Louisiana state House of Representatives, said the police needed to show "transparency" in their investigation.
"This is getting out of hand and people are clearly upset," she said.
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