The doctors on Monday also said Floyd had no underlying medical conditions that contributed to his death - and that he was likely dead before he was placed into an ambulance.

That contradicts the initial findings of the official autopsy by the Hennepin County Medical Examiner, which was cited in the court charging document against the police officer who drove his knee into Floyd's neck for several minutes.

Those initial findings said there was no evidence of traumatic strangulation. It also said coronary artery disease and hypertension also likely contributed to Floyd's death.

The county's full autopsy report has not yet been released. Later on Monday, the medical examiner declared Floyd's death was a homicide.

"The evidence is consistent with mechanical asphyxia as cause of death and homicide as manner of death," said Dr. Allecia Wilson of the University of Michigan, one of the two forensic doctors who performed an independent autopsy.

Bystander video showed Floyd pleading to be let up and saying repeatedly that he couldn't breathe as a police officer Derek Chauvin kept his knee firmly pinned into Floyd's neck for nearly nine minutes. Two other officers applied pressure with their knees to Floyd's back.

Chauvin, who is white and has been fired from the Minneapolis police department, was hit with third-degree murder and manslaughter charges last week.

But Dr. Michael Baden, who also took part in the independent autopsy at the behest of Floyd's family, said that the two other officers' actions also caused Floyd to stop breathing.

"We can see after a little bit less than four minutes that Mr. Floyd is motionless, lifeless," Baden said, adding he found no underlying health conditions in Floyd that caused his death.

Australian Associated Press