A pathologist testifying for the defense in the murder trial of former Minneapolis police officer Derek Chauvin told the court Wednesday that George Floyd died of a cardiac arrest combined with drug use, and not low oxygen as several prosecution witnesses have claimed.

Dr. David Fowler also testified that the manner of death was "undetermined" due to a combination of factors, including police restraint and carbon monoxide from a nearby squad car, that could point in multiple directions. The Hennepin County Medical Examiner's Office ruled last year that the manner of death was homicide, an act caused by another person.

"Any of the sounds Mr. Floyd is making requires you to take air in … and out …," Fowler said of bystander and police body camera videos showing Floyd pleading to breathe as three officers restrained him. "You cannot make sound unless you're … moving air and your mouth is open."

Fowler testified on the second day of the defense's case, which is expected to wrap up Thursday. It's unknown whether Chauvin will testify. Closing arguments are scheduled for next Monday, with court breaking on Friday if no additional witnesses are called.

Fowler, who recently retired after 17 years as the chief medical examiner for the state of Maryland, marked the defense's move into the cause of death phase of its case. Chauvin's attorney, Eric Nelson, has argued that Floyd likely died last May 25 of a drug overdose and pre-existing issues, including heart disease and arteries that were clogged 75% and 90% in places.

Prosecutors have argued that Chauvin cut off Floyd's air supply when he knelt on his neck as Floyd lay handcuffed stomach-down in the street with former officer J. Alexander Kueng kneeling on his buttocks and thigh area and former officer Thomas Lane kneeling and holding onto his legs.

Chauvin is on trial for second-degree murder, third-degree murder and second-degree manslaughter.

A lack of oxygen would impact the brain first, causing someone to become disoriented and speak incoherently, Fowler said.

"Mr. Floyd goes from making clear statements … and then there's a period of about forty-five seconds of silence but he's still moving … and there's a sudden relaxation" more consistent with cardiac arrest, he testified.

Fowler inadvertently opened the door for a surprising line of impromptu questioning by prosecutor Jerry Blackwell when he testified under cross-examination that Floyd experienced a "sudden cardiac arrest" and that death was not instantaneous. The moment came about after Blackwell asked him twice about his testimony on "sudden death" while being questioned by Nelson. Fowler denied using the phrase, and in doing so, introduced the time gap between cardiac arrest and death.

"Are you suggesting that though Mr. Floyd may have been in cardiac arrest there was a time when he may have been revived because he wasn't dead yet?" asked Blackwell.

"Immediate medical attention … may well reverse that process, yes," Fowler answered.

"Do you feel that Mr. Floyd should have been given immediate emergency attention to try to reverse cardiac arrest?" Blackwell asked.

"As a physician I would agree," Fowler said.

"Are you critical of the fact that he wasn't given immediate emergency care when he went into cardiac arrest?" Blackwell asked.

"As a physician I would agree," Fowler said.

Prosecutors previously told jurors that Chauvin knelt on Floyd and did not relent even though Floyd was breathless and pulseless for nearly five minutes of the approximately 9 ½-minute restraint.

The Hennepin County Medical Examiner's Office ruled Floyd's cause of death "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." It also listed hardening and thickening of the artery walls, heart disease and drug use as "other significant conditions." Fentanyl and methamphetamine were also found in Floyd's system.

At the outset of his testimony, Fowler said Floyd had a cardiac arrhythmia due to his heart disease. The doctor listed Floyd's drug use and paraganglioma, tumors that can secrete adrenaline into the body and raise blood pressure and heart rate, as factors that played a significant role.

"All of those combined to cause Mr. Floyd's death," Fowler said.

He also said Floyd's exposure to carbon monoxide from a nearby squad's exhaust contributed by reducing his oxygen-carrying capacity.

"This is just another potential insult, another brick in the wall" in the cause of death, Fowler testified.

Hennepin County Chief Medical Examiner Dr. Andrew Baker previously testified at trial that the paraganglioma in Floyd's abdomen did not play a role in his death. Nelson told jurors in his opening statement that the tumor increases the flow of adrenaline and was a factor.

Fowler said his review of Floyd's case was subjected to the scrutiny of 13 members of the Forensic Panel. Seven forensic pathologists performed a peer review, along with specialists in behavioral health, pulmonology, emergency medicine and toxicology, he said.

Fowler testified that Chauvin was likely transferring about 30 to 35 pounds of his 140-pound body weight onto Floyd, and that it was not enough pressure to leave any bruises on Floyd's neck or upper back, according to Baker's autopsy.

"All of [Floyd's] injuries were in the areas where [Chauvin's] knee was not," Fowler said.

Fowler testified that prone restraint was not as dangerous as prosecution witnesses have claimed, and that one study called it an "interesting hypothesis" unsupported by data.

"The prone position itself does not have … or there is no evidence right now that it's a significant condition," Fowler said, noting that it is used to improve oxygen intake for some COVID-19 patients.

Fowler said there were "conflicting manners" concerning Floyd's death. "Restrained in a very stressful situation" would be homicide, he said. Ingesting drugs would be accidental, and there were the natural causes of Floyd's heart problems and other health challenges, he said.

Therefore, Fowler said, "I would fall back to undetermined in this particular case."

Blackwell suggested that Fowler tried to confuse jurors and asked him several clarifying questions, getting Fowler to confirm that he has never seen any test results showing that Floyd was injured by carbon monoxide, and that the squad car was a hybrid, which operates with both an electric motor and gasoline engine. Fowler also acknowledged under cross-examination that he is not a toxicologist or cardiologist, has not measured anyone's breathing and does not treat living patients.

"Do you agree, Dr. Fowler, that the majority of cases where somebody dies of asphyxiation are very similar and no traumatic manifestations are visible at all?" Blackwell asked.

"That is correct, depending on the circumstances," Fowler said.

Fowler's testimony comes as he and several others face a lawsuit filed in federal court by the family of Anton Black, a young Black man who died in police custody in 2018 in Maryland.

The suit, which did not come up at Chauvin's trial, alleges that Black's death was caused by officers' excessive use of force and that Fowler and other public officials covered up for the officers. The suit contends that Fowler unnecessarily delayed the release of autopsy results and was unethically influenced by police. No one was charged in Black's death.

"Two years before George Floyd died after being restrained and pinned down by police, 19-year-old Anton Black … was killed by three white law enforcement officials and a white civilian in a chillingly similar manner on Maryland's Eastern Shore," the suit reads, according to the Washington Post.

Kueng, Lane and former officer Tou Thao, who kept angry bystanders at bay, are scheduled to be tried Aug. 23 with aiding and abetting second-degree murder and manslaughter in Floyd's death.

All four defendants, who were fired, are out on bond.