The Minneapolis Police Department is embroiled in a new racially charged death controversy less than a year after former officer Mohamed Noor was sentenced to 12.5 years' prison for murdering Australian life coach Justine Damond.
The latest incident in the US midwest city was live-streamed on Facebook by a bystander.
The video shows a white police officer, Derek Chauvin, arresting an African-American man, George Floyd, in a Minneapolis street.
Chauvin is kneeling on Floyd's neck.
Floyd is handcuffed, can be heard protesting he could not breathe, becomes motionless and dies.
Minneapolis mayor Jacob Frey and civil rights groups condemned the incident.
"Being black in America should not be a death sentence," Frey told reporters on Tuesday.
"For five minutes, we watched a white officer press his knee into a black man's neck.
"When you hear someone calling for help, you're supposed to help."
Police said Floyd had matched the description of a suspect in a forgery case at a grocery store and accused him of resisting arrest.
Chauvin and three other officers at the scene were fired on Tuesday and protests and vigils were being organised.
The FBI will investigate Chauvin for possible civil rights violations.
Minnesota law enforcement authorities are also investigating.
Other recent racially charged fatal police shootings in the Minneapolis-St Paul twin cities include two white police officers shooting dead 24-year-old Jamar Clark during a 2015 scuffle.
The police officers were not charged.
In 2017 a police officer was acquitted after shooting dead Philando Castile during a traffic stop while Castile's girlfriend live-streamed the incident.
Damond, 40, originally from Sydney's northern beaches, was shot dead by Somali-born police officer Noor.
Damond was in her Minneapolis home just before midnight on July 15, 2017, when she heard a woman's screams near the alley behind her house.
Fearing the woman was being raped, Damond called 911 and when Noor and his partner, Mathew Harrity, arrived in the alley she approached their police vehicle.
Damond was dressed in her pyjamas.
Noor, in the front passenger seat and fearing he was being ambushed, shot across Officer Harrity, out the open driver's window, and fatally struck Damond in the stomach.
A Minneapolis jury found Noor, 34, guilty last year of third-degree murder and second-degree manslaughter.
Noor has filed an appeal.
Damond's Sydney-based family launched a $US50 million ($A75 million) civil lawsuit against Minneapolis last year and just days after Noor's conviction the city agreed to pay $US20 million ($A30 million).