A video of a handcuffed black man dying while a Minneapolis officer knelt on his neck for more than five minutes has sparked an outcry in the US over police treatment of African Americans.
On Wednesday US President Donald Trump ordered an investigation into his death.
It comes after Minneapolis mayor Jacob Frey dismissed four police officers following the death in custody of George Floyd on Monday as the suspect was pressed shirtless on to a Minneapolis street, one officer's knee on his neck.
"Your knee in my neck. I can't breathe... Mama. Mama," Mr Floyd pleaded.
Bystanders filmed the scene as Floyd, thought to be in his 40s, slowly grew silent and motionless, unable to move even as the officers taunted him to "get up and get in the car".
He was taken to hospital where he was later declared dead.
Mr Frey expressed outrage as calls grew for the officers to be prosecuted for murder.
"What I saw was wrong at every level," he said of the video.
"For five minutes, we watched as a white officer pressed his knee into the neck of a black man," Mr Frey said.
"Being black in America should not be a death sentence."
Mr Trump said "justice will be served".
"At my request, the FBI and the Department of Justice are already well into an investigation as to the very sad and tragic death in Minnesota of George Floyd," he tweeted.
"I have asked for this investigation to be expedited and greatly appreciate all of the work done by local law enforcement. My heart goes out to George’s family and friends. Justice will be served!"
Civil rights attorney Ben Crump said he had been retained by Mr Floyd's family.
Mr Crump said Mr Floyd had been stopped by police over a forgery accusation, a charge often used for writing bad cheques or using fake banknotes for purchases.
"This abusive, excessive and inhumane use of force cost the life of a man who was being detained by the police for questioning on a non-violent charge," he said.
Mr Floyd's death recalled the 2014 choking death of New Yorker Eric Garner by police, who was being detained for illegally selling cigarettes.
His death helped spark the nationwide Black Lives Matter movement.
Minneapolis Police chief Medaria Arradondo said he had passed the case to the FBI for investigation, which could turn it into a federal rights violation case.
But there were mounting calls for the officers' arrest on homicide charges.
"This is pure evil," said Nekima Levy Armstrong, an African American Minneapolis civil rights attorney.
"Those same officers need to be charged and convicted of murder," she said.
Mr Floyd's death comes on the heels of two other deaths of African-Americans that involved police wrongdoing.
On March 13 in Louisville, three white Kentucky policemen forced their way into the home of a black woman, Breonna Taylor, and shot her in a drug investigation.
And police and prosecutors in Brunswick, Georgia, allegedly covered up the killing of a young black jogger by the son of a retired investigator for local law enforcement.
The police allegedly withheld for two months a video showing Ahmaud Arbery, 25, being followed and then shot with a shotgun in broad daylight.
The families of Arbery and Taylor are also being represented by Mr Crump.
The American Civil Liberties Union said the Minneapolis case showed that US police continue to use harsh treatment on African Americans accused of minor charges.
"This tragic video shows how little meaningful change has emerged to prevent police from taking the lives of black people," said ACLU policing specialist Paige Fernandez.
"Even in places like Minneapolis, where chokeholds are technically banned, black people are targeted by the police for low-level offences and are subjected to unreasonable, unnecessary violence," she said.