MINNEAPOLIS (AP) — George Floyd died from a lack of oxygen, which damaged his brain and caused his heart to stop, a medical expert testified Thursday at former Officer Derek Chauvin's murder trial.

Floyd's breathing was too shallow to take in enough oxygen while he was pinned facedown with his hands cuffed behind his back for 9 1/2 minutes as Chauvin knelt on his neck and back, said Dr. Martin Tobin, a lung and critical care specialist at the Edward Hines, Jr. VA Hospital and Loyola University's medical school in Chicago.

His testimony comes a day after a use-of-force expert testified that the now-fired white officer bore down with most of his weight on Floyd's neck or neck area and his back the entire 9 1/2 minutes.

Jody Stiger, a Los Angeles Police Department sergeant serving as a prosecution witness, said Wednesday that based on his review of video evidence, Chauvin applied pressure to Floyd's neck or neck area from the time officers began pinning Floyd to the ground until paramedics began to move him to a stretcher.

“That particular force did not change during the entire restraint period?” prosecutor Steve Schleicher asked as he showed the jury a composite of five still images.

“Correct,” replied Stiger, who on Tuesday testified Chauvin used excessive force against Floyd.

Chauvin attorney Eric Nelson countered by pointing out what he said were moments in the video footage when Chauvin's knee did not appear to be on Floyd's neck but on his shoulder blade area or the base of his neck. Stiger did not give much ground, saying the officer's knee in some of the contested images still seemed to be near Floyd's neck, though he agreed his weight might have shifted at times.

In other testimony, the lead Minnesota state investigator on the case, James Reyerson, initially agreed with Nelson that Floyd seemed to say in a police body-camera video of his arrest, “I ate too many drugs.”

But when a prosecutor played a longer clip of the video, Reyerson said he believed what Floyd really said was “I ain't do no drugs.”

Chauvin, 45, is charged with murder and manslaughter in Floyd’s death May 25. Floyd, 46, was arrested outside a neighborhood market after being accused of trying to pass a counterfeit $20 bill. A panicky-sounding Floyd struggled and claimed to be claustrophobic as police tried to put him in a squad car, and they pinned him to the pavement.

Bystander video of Floyd crying that he couldn’t breathe as onlookers yelled at Chauvin to get off him sparked protests and scattered violence around the U.S.

Nelson has argued that the now-fired white officer “did exactly what he had been trained to do over his 19-year career,” and he has suggested that Floyd's drug use and his underlying health conditions are what killed him, not Chauvin’s knee, as prosecutors contend. Fentanyl and methamphetamine were found in Floyd's system.

Breahna Giles, a state forensic scientist, testified Wednesday that pills found in the SUV Floyd was driving contained methamphetamine and fentanyl. Another witness, forensic chemist Susan Meith, testified that remnants of a pill found in the back of the police squad car also contained methamphetamine and fentanyl. Earlier testimony revealed that that pill contained DNA from Floyd’s saliva.