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.
It comes as US President Donald Trump urged state governors to crack down on protests that have engulfed cities as officials extended curfews to prevent a seventh night of looting and vandalism.
Residents and business owners in cities from New York to Santa Monica, California, spent Monday sweeping up broken glass and taking stock of damage after protests over excessive police force against African Americans turned violent again overnight.
“You have to dominate,” Trump told the governors in a private call obtained by media including Reuters. “If you don’t dominate, you’re wasting your time – they’re going to run over you, you’re going to look like a bunch of jerks.”
Trump said the federal government was going to clamp down “very strong” on the violence.
Dozens of cities across the United States remain under curfews at a level not seen since riots following the 1968 assassination of civil rights activist Martin Luther King Jr.
The National Guard deployed in 23 states and Washington DC.
Curfews were extended in Washington DC, where authorities fought to put out fires near the White House overnight; in Minnesota, which has experienced some of the worst violence, and in Los Angeles.
One person was killed in Louisville, Kentucky, overnight where police and National Guard troops returned fire while trying to disperse a crowd.
Police in Chicago, the country’s third-largest city, fielded more than 10,000 calls for looting, Mayor Lori Lightfoot told a briefing.
The unrest, which erupted as the country was easing lengthy lockdowns to stop the spread of the coronavirus, began with peaceful protests over the death of a black man, George Floyd, in police custody in Minneapolis last Monday.
Video footage showed a white police officer kneeling on the neck of Floyd, 46, for nearly nine minutes before he died.
Derek Chauvin, a since-fired 44-year-old police officer, has been charged with third-degree murder and second-degree manslaughter.
He was released on $US500,000 ($A735,710) bail and is due to appear in court on June 8, according to jail records.
On Monday, dozens of people quietly paid their respects to Floyd at the scene outside the Cup Foods where he lost his life. Visitors left flowers and signs honoring Floyd on the pavement.
Terrence Floyd, the victim’s brother, told the gathering he wanted people to get educated, vote and not destroy their own communities.
“Let’s do this another way,” he said.
Floyd’s death was the latest in a string of similar incidents to prompt an outcry over racism in law enforcement.
It reignited outrage across a politically and racially divided country that has been hit hard by the coronavirus pandemic, with African Americans accounting for a disproportionately high number of coronavirus cases.
The US Justice Department has directed the Bureau of Prisons to send riot-control teams to Miami and Washington DC, to help manage the protests, a senior department official told reporters.
Department investigators are interviewing people arrested during protests who might face federal charges for such offences as crossing state lines to incite violence, the official said.
Many cities affected by the unrest are restarting some normal economic activity after more than two months of stay-at-home orders to stem a pandemic that has killed more than 104,000 people and plunged more than 40 million people into joblessness.
New York City Police Commissioner Dermot Shea told a news briefing with Mayor Bill de Blasio that police are seeing “outside agitators coming and trying to rally people to do bad things”.
Trump has condemned the killing of Floyd and promised justice but has made no major public statement to address the crisis. In tweets he has described violent protesters as “thugs” and threatened to use the US military.
A US congressman plans to introduce legislation this week to eliminate a legal doctrine that protects police officers from being sued for illegal and unconstitutional acts.
US Representative Justin Amash, a libertarian independent from Michigan, won support from a Minneapolis Democrat on Monday for his “Ending Qualified Immunity Act,” which would allow civil lawsuits against police, a recourse that the Supreme Court has all but done away with.
The high court’s adoption of the qualified immunity doctrine has largely shielded police and other government officials from having to pay financial settlements to victims or grieving families.
The doctrine increasingly protects cops even when courts have determined that the officers violated a victim’s civil rights.
“The brutal killing of George Floyd is merely the latest in a long line of incidents of egregious police misconduct,” Amash told colleagues in a letter posted to Twitter on Sunday.
“This pattern continues because police are legally, politically and culturally insulated.”
“That must change so that these incidents stop happening.”
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