Disturbing video shows a black man dying in police custody in Minneapolis on Monday night after he repeatedly said "I can't breathe" as a white police officer pushed a knee into his neck.

In a video posted to Facebook by an onlooker, the man can be seen talking to officers and onlookers, begging for help, as two officers press him into the ground, before he becomes unresponsive.

"Please, please, I can't breathe. Please, man, please," the man says repeatedly, as a police officer holds a knee against his neck for eight minutes.

"I can't breathe," says the man, who has not yet been officially identified.

A person can be heard talking to him, telling him to get up and get in the car, although it is unclear if it is an officer speaking.

"I can't move," says the man. "Everything hurts. Give me some water or something, please. I can't breathe, officer."

"They're going to kill me," he says. "They're going to kill me, man."

Onlookers can be heard calling on police to stop, stating the man's nose was bleeding, he was not resisting arrest, and that the officer was stopping his breathing.

"He's fine," responds one officer, saying they'd tried to put him into a car for 10 minutes.

When the man being detained becomes unresponsive, onlookers start demanding officers give the man medical attention.

"He's not responsive right now," says one onlooker, who goes forward to help as a police officer pulls out a can of mace.

"Does he have a pulse?" asks another onlooker. "Check his pulse!"

The officers repeatedly tell the onlookers to get back and do not immediately provide medical attention for the unresponsive man in their custody.

Paramedics then arrive, check the unresponsive man's pulse, and roll him onto a gurney and into an ambulance.

Police told local station KSTP the man was taken to Hennepin County Medical Center, where he died.

The incident took place in front of a supermarket at 38th Street and Chicago Avenue in Minneapolis.

Minneapolis police did not immediately respond to BuzzFeed News' request for comment, but in a statement to KSTP, police spokesperson John Elder said the man died after "medical distress."

"He was ordered to step from the car ... after he got out he physically resisted officers ... officers were able to get the suspect into handcuffs and the officers noticed the male was going into medical distress," Elder told KSTP.

"They are literally lying," wrote the woman who posted the Facebook video of the incident.

The FBI is now investigating the incident.

In an emotional press conference on Tuesday morning, Minneapolis Mayor Jacob Frey said the man "should not have died" and that the officer "failed in the most basic human sense."

"What we saw was horrible, completely and utterly messed up," said Frey.

"Being black in America should not be a death sentence," Frey said. "For five minutes, we watched as a white officer pressed his knee into the neck of a black man. For five minutes. When you hear someone calling for help, you are supposed to help. This officer failed in the most basic human sense."

The man's death is the latest in a long history of unarmed black people dying in interactions with police in the US.

The Minneapolis man's plea of "I can't breathe" was the same as Eric Garner, who died in New York City in 2014 after an NYPD officer placed him in an illegal chokehold. Video taken by an onlooker also captured his death.

In 2015, there were widespread protests in Minneapolis when 24-year-old Jamar Clark was shot dead by two white police officers. Authorities later declined to charge the officers, ruling their use of deadly force was justified.

Minneapolis police also came under scrutiny in 2017 when a black officer shot dead an unarmed white woman, who had called 911 to report a possible sexual assault. The officer is now serving a 12-year prison sentence.

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