George Floyd, 46, died handcuffed on the street in the custody of police in Minneapolis on Memorial Day. An officer responding to a report that Floyd had tried to pass a fake $20 bill held him down by lodging a knee on his neck for nearly nine minutes.
He died shortly after lapsing into unconsciousness.
A 10-minute cellphone video of the encounter ignited protests across the country, not only over Floyd's death, but over the bigger question of police brutality and entrenched racial injustice.
All four officers involved in the arrest have been fired, and the officer who was recorded kneeling on Floyd's neck, Derek M Chauvin, has been charged with third-degree murder and second-degree manslaughter.
Still unanswered is the question of precisely what caused Floyd's death. On Monday, lawyers for Floyd's family said a private autopsy had concluded that the death was a homicide, brought about by two officers kneeling on his neck and his back.
The result was a constriction of blood to the brain and air to the lungs, two doctors found in the private autopsy.
"The autopsy shows that Floyd had no underlying medical problem that caused or contributed to his death," said one of the private examiners, Dr Michael Baden, who was the chief medical examiner for the City of New York in the late 1970s.
But officials in Minneapolis have not released the results of the official death examination.
What do we know about how Floyd died?
The video showed Chauvin with his knee on the back of Floyd's neck for eight minutes and 46 seconds. Floyd was unresponsive for nearly three minutes of that. He repeatedly gasped, "I can't breathe," but a criminal complaint in the case said he had started to say he couldn't breathe while he was still standing.
The full report of the Hennepin County medical examiner is pending, but the criminal complaint said the autopsy "revealed no physical findings that support a diagnosis of traumatic asphyxia or strangulation". Floyd, the complaint said, had underlying health conditions, including coronary artery disease and hypertensive heart disease.
"The combined effects of Floyd being restrained by the police, his underlying health conditions and any potential intoxicants in his system likely contributed to his death," the complaint said.
But the private doctors said their findings contradict that.
How can there have been no asphyxia?
Lawrence Kobilinsky, a forensics expert at John Jay College, said that such a finding means there was no apparent damage to the internal structures of Floyd's neck, such as damage to delicate bones and cartilage. But that does not mean the police officer's actions did not lead to his death, he said.
"They have to come up with another explanation," he said. "It does seem to me though, if the officer didn't have his knee on his neck and being in prone position and with other officers holding him down, restraining him the way he was, had they not done that, he would be well and alive."
The private autopsy concluded that even without findings of "traumatic" asphyxia, such as broken bones, the compression caused by the officers still led to Floyd's death by depriving his brain of blood and his lungs of air.
When is the official autopsy report being released?
The Hennepin County Medical Examiner's office said it was awaiting final results from laboratory studies "to provide the most medically accurate cause of death determination possible."
"The autopsy alone cannot answer all questions germane to the cause and manner of death," its statement said, but must be "interpreted in the context of the pertinent investigative information and informed by the results of laboratory studies."
What other medical issues are under review?
The autopsy results should show whether Floyd was under the influence of alcohol or any other drug.
Toxicology results usually take weeks, but presumably the testing will be expedited in this high-profile case.
Another thing that could come up: According to the complaint, one of the police officers at the scene said Floyd might have been experiencing "excited delirium", a condition characterised by agitation, aggression, acute distress and sudden death.
It is usually associated with drug users who suddenly die in law enforcement custody, though not all experts accept it as a real cause of death.
What do we know about the doctors who conducted the autopsy for the family?
One of the two doctors was Baden, best known as host of the HBO television show Autopsy. He also did the private autopsy in the case of Michael Brown, who was killed by a police officer in 2014 in Ferguson, Missouri.
Baden was the first to reveal that Brown, an unarmed black teenager, had been shot at least six times, including twice in the head.
Also participating in the private autopsy was Dr Allecia Wilson, director of autopsy and forensic services at the University of Michigan.
In a news conference on Monday, she concurred with Baden, saying, "The evidence is consistent with mechanical asphyxia as the cause of death and homicide as the manner of death."
What questions remain?
Glenn A Zeitz, a Philadelphia trial lawyer specialiing in civil rights, said a key question will be whether the official medical examiners find that the cause of death was the combined effect of any preexisting heart condition and coronary artery disease.
It remains to be seen whether the private autopsy results will differ substantially from those in the official report.
"You can have two people looking at the same thing coming up with different explanations," Kobilinsky said. "This is not necessarily science, it's medicine. Medicine is an art and a science."
Frances Robles and Audra DS Burch c.2020 The New York Times Company