In the ongoing murder trial of police officer Derek Chauvin, a respiratory expert on Thursday testified that George Floyd died from lack of oxygen and that police Derek Chauvin‘s knee was on his neck “more than 90% of the time.”

Dr Martin Tobin, a pulmonologist, told the jury at Chauvin’s murder and manslaughter trial that he had watched videos of Floyd’s May 25, 2020 arrest “hundreds of times.”

“Mr. Floyd died from a low level of oxygen,” Tobin told the nine-woman, five-man jury hearing the high-profile case in a heavily guarded Minneapolis courtroom.

“This caused damage to his brain,” he said, and arrhythmia — an irregular heartbeat — which “caused his heart to stop.”

The 45-year-old Chauvin, who is white, was seen in a video taken by a bystander kneeling on the neck of a handcuffed Floyd for more than nine minutes as the 46-year-old African-American man complained repeatedly that he “can’t breathe.”

The video of Floyd’s arrest touched off protests against racial injustice and police brutality in the United States and around the world.

Tobin said Floyd’s breathing weakened because he was face down on the street, handcuffed and with Chauvin and other officers on his neck and back.

Eric Nelson, Chauvin’s defense attorney, has suggested at several points during the trial that Chauvin’s body weight was actually on Floyd’s shoulder or back at times and not on his neck.

Tobin disagreed.

“Officer Chauvin’s knee is virtually on the neck for the vast majority of the time,” he said, “more than 90% of the time.”

The Irish-born Tobin is testifying as an expert witness for the prosecution.

The Chicago-based doctor said he has testified previously at medical malpractice trials but this is his first criminal trial and he is not being paid.

Chauvin, who has pleaded not guilty, faces up to 40 years in prison if convicted of the most serious charge — second-degree murder.

Chauvin, a 19-year-old veteran of the Minneapolis Police Department, was fired from the force after Floyd’s death.