The death of 37-year-old Alton Sterling outside a convenience store in Baton Rouge was captured on cell phone video, triggering protests in the city and outrage nationwide as the United States struggles to respond to alleged police brutality against black suspects.
It came on the eve of a closely watched trial in Baltimore for an officer charged in the death of Freddie Gray, a black man whose spine broke last year while he was transported in a police van.
His family's lawyer said Sterling was merely selling CDs outside the Triple S store, with the permission of the shop's owner.
In Baton Rouge, Louisiana Governor John Bel Edwards promised a full investigation into the "disturbing" incident, with the Department of Justice civil rights division taking the lead, assisted by the Louisiana US Attorney's office and the FBI.
"I have very serious concerns," Edwards told a news conference. "The video is disturbing to say the least."
At an emotional press conference held earlier with relatives of the victim, the head of the civil rights group NAACP in Baton Rouge, Michael McClanahan, called for the resignations of the police chief and mayor.
"What we are going to do today is rule out the one per cent of bad police officers that go around becoming the judge, the jury, the executioner of people period but more specifically, innocent black lives," he said.
Edmond Jordan, an attorney representing Sterling's family, had demanded the investigation be carried out by an neutral third party "to ensure that there is no cover-up."