George Floyd, the African-American man whose death has sparked civil unrest, died from asphyxia (lack of oxygen), a private post-mortem examination found.
He died due to compression on his neck and back by Minneapolis police officers, medical examiners hired by the Floyd family said.
The findings differ from an official preliminary examination carried out by the county medical examiner.
It did not find evidence of “traumatic asphyxia or strangulation”.
The official examination also said underlying health conditions played a role in Mr Floyd’s death.
But the two doctors hired by the Floyd family found the death was a homicide, a statement from its legal team said.
“The cause of death in my opinion is asphyxia, due to compression to the neck – which can interfere with oxygen going to the brain – and compression to the back, which interferes with breathing,” Dr Michael Baden, a former New York City medical examiner and one of the pair, said at a news conference.
A video showing a white police officer continuing to kneel on George Floyd’s neck even after he pleaded he could not breathe sparked outrage when it emerged a week ago.
It has led to six consecutive days of protests around the United States and a level of civil unrest not seen this widely across the country in decades.
Benjamin Crump, a lawyer for the Floyd family, told Monday’s news conference: “Beyond doubt he would be alive today if not for the pressure applied to his neck by officer Derek Chauvin and the strain on his body by two other officers.”
He added: “The ambulance was his hearse.”
Dr Baden said there was “no other health issue that could cause or contribute to the death”.
The findings contradict those of a preliminary post-mortem examination that was included in the criminal complaint against Mr Chauvin, who has been charged with third-degree murder and manslaughter.
The medical examiner noted in that report that Mr Floyd had underlying heart conditions and said the combination of these, “potential intoxicants in his system” and being restrained by the officers “likely contributed to his death”.
The full official death examination is yet to be released by the office of the Hennepin County Medical Examiner. It says it is awaiting more results from laboratory studies.
The Floyd family and their lawyers say the charge against Mr Chauvin should be increased to first-degree murder. They say the private post-mortem examination proves two other officers filmed kneeling on his back also contributed to his death.