The death Monday night after a struggle with officers was under investigation by the FBI and state law enforcement authorities.
The man who died was identified as George Floyd by Ben Crump, a prominent civil rights and personal injury attorney who said he had been hired by Floyd's family.
Floyd is from Houston and went to Jack Yates High School, where he played football, according to friends and former classmates.
"We all watched the horrific death of George Floyd on video as witnesses begged the police officer to take him into the police car and get off his neck," Crump said in a statement. "This abusive, excessive and inhumane use of force cost the life of a man who was being detained by the police for questioning about a non-violent charge."
Police said Floyd matched the description of a suspect in a forgery case and resisted arrest. The unidentified officer ignored his pleas. "Please, please, please, I can't breathe. Please, man," Floyd was heard telling the officer.
After several minutes, one of the officers tells him to "relax." Minutes pass, and Floyd becomes motionless under the officer's restraint. The officer leaves his knee on Floyd's neck for several minutes more.
Floyd worked as a security guard at Conga Latin Bistro in Minneapolis. Jovanni Thunstrom, who is both Floyd's employer and landlord, told ABC News that Floyd has a daughter in Houston who he was planning to bring to Minneapolis to live with him.
The incident drew comparisons to the case of Eric Garner, an unarmed black man who died in 2014 in New York after he was placed in a chokehold by police and pleaded for his life, saying he could not breathe.
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Minneapolis Mayor Jacob Frey apologized to the black community Tuesday in a post on his Facebook page.
"Being Black in America should not be a death sentence. For five minutes, we watched a white officer press his knee into a Black man's neck. Five minutes. When you hear someone calling for help, you're supposed to help. This officer failed in the most basic, human sense," Frey posted.
Frey later tweeted the four responding officers have been terminated, adding, "This is the right call."
Several witnesses had gathered on a nearby sidewalk, with some recording the scene on their phones. The bystanders became increasingly agitated as the man pleaded with police. One bystander tells officers they need to let him breathe. Another yells at them to check the man's pulse.
Black man dies after video shows officer kneeling on neck
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Asked by reporters about the use of the knee on the man's neck, Minneapolis Police Chief Medaria Arradondo said the department has "policies in place regarding placing someone under control" that "will be part of the full investigation we'll do internally."
Two experts on police use of force said the officer clearly restrained the man for too long.
The man was under control and no longer fighting, said David A. Harris, a University of Pittsburgh law professor who specializes in police conduct.
Andrew Scott, a former Boca Raton, Florida, police chief who now testifies as an expert witness in use-of-force cases, agreed, saying the officers should have at least rolled him on his side so he could breathe.
The death was "a combination of not being trained properly or disregarding their training," Scott said. "He couldn't move. He was telling them he couldn't breathe, and they ignored him. I can't even describe it. It was difficult to watch."
The New York City officer in the Garner case said he was using a legal maneuver called "the seatbelt" to bring down Garner, whom police said had been resisting arrest. But the medical examiner referred to it as a chokehold in the autopsy report and said it contributed to his death. Chokehold maneuvers are banned under New York police policy.
A grand jury later decided against indicting the officers involved in Garner's death, sparking protests around the country.
In Minneapolis, kneeling on a suspect's neck is allowed under the department's use-of-force policy for officers who have received training in how to compress a neck without applying direct pressure to the airway. It is considered a "non-deadly force option," according to the department's policy handbook.
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A chokehold is considered a deadly force option and involves someone obstructing the airway. According to the department's use-of-force policy, officers are to use only an amount of force necessary that would be objectively reasonable.
The police union asked the public to wait for the investigation to take its course and not to "rush to judgment and immediately condemn our officers." The Hennepin County Attorney's Office, which would handle any prosecution of police on state charges, said in a statement that it was "shocked and saddened" by the video and pledged to handle the case fairly. The U.S. Attorney's Office in Minnesota declined comment.
The death came amid outrage over the death of Ahmaud Arbery, who was fatally shot Feb. 23 in Georgia after a white father and son pursued the 25-year-old black man they had spotted running in their subdivision. More than two months passed before charges were brought. Crump also represents Arbery's father.
All body camera footage in the Minneapolis case has been turned over to the Minnesota Bureau of Criminal Apprehension, and the agency asked to speak with anyone who saw the arrest or recorded video. The officers involved have been put on paid administrative leave, per department protocol. The agency said the officers' names will be released after initial interviews with the people involved and witnesses.
The FBI is conducting a separate federal civil rights investigation, at the request of Minneapolis police, the BCA said. Messages left with the FBI were not immediately returned.
The Associated Press contributed to this report.