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An expert witness in the Derek Chauvin trial on Wednesday backed the defense's contention that George Floyd died of cardiac arrest combined with illicit drugs in his body and not from a lack of oxygen while pinned to the pavement by the now-fired police officer for more than 9 minutes late last spring.

The testimony of Dr. David Fowler, who recently retired after 17 years as the chief medical examiner for the state of Maryland, started the cause of death phase of the defense's case. Fowler wrapped up his testimony by saying he would have classified Floyd's manner of death as undetermined due to the multiple factors he cited.

During the afternoon and under cross examination by prosecutor Jerry Blackwell, Fowler endorsed the state's contention that Chauvin and the other officers were negligent in failing to provide lifesaving measures to Floyd once he lacked a pulse roughly 5 minutes into being restrained.

"As a physician, I would agree," Fowler said when asked whether Floyd should have been given immediate medical attention at that point.

"Are you critical that he wasn't given immediate medical attention?" Blackwell then asked. Again, Fowler responded, "As a physician, I agree."

Defense attorney Eric Nelson rose again to question Fowler further late in the day's proceedings but did not address Fowler's opinion on that exchange, which scored points for the prosecution.

At the outset of Fowler's testimony, he said Floyd had a cardiac arrhythmia due to his heart disease during his restraint by police while prone on the pavement at 38th and Chicago. The doctor said Floyd's fentanyl and methamphetamine use also played a significant role. He also included exposure to possible vehicle exhaust from a police squad close by his paraganglioma, tumors that can form near the carotid artery, and along nerve pathways in the head and neck.

"All of those combined to cause Mr. Floyd's death," Fowler said. He did not include lack of oxygen, or asphyxia, as the prosecution contended during the two-plus weeks it presented evidence.

Much of what Fowler pointed to is largely the same as the findings of Dr. Andrew Baker, the Hennepin County medical examiner. However, Baker previously testified in the trial that paraganglioma played no role in Floyd's death. Nelson said in his opening statement that such a tumor increases the flow of adrenaline.

Fowler said his review of Floyd's case was subjected to scrutiny by 13 members of The Forensic Panel, a for-profit group of medical professionals. He said there were seven forensic pathologists among the 13 who provided peer review, along with specialists in behavioral health, pulmonology, emergency medicine and toxicology.

Fowler's testimony came as he is among several people being sued in federal court on behalf of the family of Anton Black, a young Black adult who died in police custody in September 2018 in Maryland.

The suit alleges that Black's death was caused by officers' excessive use of force and that Fowler and other public officials covered up for those 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.

"Anton Black died because police employed excessive force, laying him out prone on his stomach, lying on top of him for approximately six minutes and approximately five minutes after he was handcuffed, and folding his legs toward the sky in a manner that further compromised his ability to breathe," the lawsuit continues, the Post reported.

LaToya Holley, Black's sister, told WTOP Radio in Washington, D.C., this week, "It's surreal that you have two men on the opposite sides of the country that experienced almost the same treatment by two different police officers. The medical examiner [Fowler], in my opinion, was egregious in the way he finalized Anton's autopsy results. Now, he's being called to be an expert witness for another police officer."

The lawsuit did not come up during Fowler's time on the witness stand or at any other time during the proceedings, which concluded about 45 minutes sooner than usual. The trial is scheduled to resume between 9:15 and 9:30 a.m. Thursday with additional defense testimony.

After Wednesday's morning break in the Chauvin trial, Fowler explained why he added carbon monoxide poisoning as a possible contributor to Floyd's death even though he found no evidence the autopsy ran tests for the toxic gas.

Fowler said that video from an officer's body-worn camera showed Floyd's face pointed toward an exhaust pipe of a running police squad.

The doctor testified that it doesn't take much carbon monoxide saturation, as low as 6%, to begin showing effects in young healthy individuals, let alone people with underlying health conditions such as Floyd.

"In Mr. Floyd, it robs him of an additional percentage of oxygen carrying capacity, whether it's 5% or 10%, it takes away from the important factor of getting blood to his heart," he said. "This is another insult, another brick in the wall" of possible contributing factors, he said, adding that the officers holding down Floyd were farther from the source presumed healthier and less likely to be affected.

Earlier testimony revealed that the vehicle was a hybrid, which operates with both an electric motor and gasoline engine.

Fowler also cited a study that concluded that no individuals were found to suffer from asphyxia in the so-called "hog tie position." Weights were applied to individuals totaling up to 225 pounds, and the study still found no disturbance in their ability to breathe. However, he acknowledged the subjects of the study were placed under different circumstances than Floyd such as on a softer floor surface and knowing that their lives were not at risk.

In connection with having Chauvin and other officers on top of him, Fowler also testified that he could find no bruises or other injuries to the back. He also said he concluded that Chauvin's left knee was "nowhere close to the airway" at anytime.

"Any of the sounds Mr. Floyd is making requires you to take air in ... and out," the doctor. "You cannot make sound unless you're ... moving air and your mouth is open."

Floyd was "coherent and understandable" and not confused, incoherent or disoriented before "very rapidly" becoming silent and motionless, Fowler said. He went on to call Floyd's transition "much more consistent with a sudden cardiac event vs. low oxygen, which affects the brain first."

Fowler spelled out the health risks of taking the powerful opioid fentanyl as well as methamphetamine. Both drugs were found in Floyd's body during autopsy. The doctor also said his review of toxicological tests on Floyd showed the meth use was "recent."

The doctor said there were "conflicting manners" concerning Floyd's death. "Restrained in a very stressful situation," that 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."

Under cross examination, prosecutor Blackwell suggested that Fowler was expressing bias, cherry-picking the facts to confuse the jury and jumping to conclusions when he testified about the possibility of carbon monoxide playing a role in Floyd's death and at other times when questioned by Nelson.

"Going right to the punch line on carbon monoxide," Blackwell said, "you haven't seen any data or test results that showed Mr. Floyd had a single injury from carbon monoxide, correct?"

Fowler replied, "It is true."

"As you were talking about carbon monoxide, you were referring to the squad car that Mr. Floyd was near? Have you ever laid eyes on the squad car you were referring to?"

Fowler said he had not but knew it had four exhaust pipes.

"How do you know the car was even on?" Blackwell said, to which Fowler said he noted water on the pavement that could have come from the vehicle's exhaust.

Blackwell accused Fowler of assuming about the water's origin, and Fowler countered, "It's not an assumption; it's an evaluation … that the car was running."

Referring to Fowler classifying Floyd's death as occurring "very rapidly," Blackwell asked the doctor whether he documented in his analysis of the evidence the time of death.

"I don't specifically remember doing that," Fowler said. "There's not going to be a period time in this particular case. … The moment of death is not something you can easily document."

Blackwell also elicited testimony that Floyd didn't complain of a headache, a common symptom of a paraganglioma — or abnormal cell cluster — secreting adrenaline, as Fowler contended. He also noted how Floyd complained of being choked and that he never fell sleepy or had a slowed heart rate — signs of an overdose.

Chauvin is charged with second-degree unintentional murder, third-degree murder and second-degree manslaughter in the killing of Floyd last spring. Three other fired officers who assisted in Floyd's arrest — J. Alexander Kueng, Thomas Lane and Tou Thao — are scheduled to be tried in August on charges of aiding and abetting murder and manslaughter.

Before testimony started Wednesday, Hennepin County District Judge Peter Cahill ruled that a friend who was with Floyd at the time of his arrest can refuse to testify. Morries L. Hall, 42, invoked his constitutional right against self-incrimination and told the judge that he fears being charged with serious crimes should he answer questions from the defense or prosecution.

"I am fearful of criminal charges going forward," Hall said while standing at the podium usually reserved for attorneys. "I [also] have open charges that's not settled yet."

His attorney told the court last week that Hall would open himself up to third-degree murder charges in connection with Floyd's death if he were to testify.

Cahill said, "I'm finding that he has a complete Fifth Amendment privilege here" and then quashed the subpoena, a ruling that was a defeat for the defense's effort to strengthen its contention that fentanyl and methamphetamine played a role in Floyd's death.

Hall is suspected of being a provider of illicit drugs to Floyd over an extended period of time. Floyd's girlfriend testified earlier that Hall supplied Floyd with drugs sometime in the month of Floyd's death.

Star Tribune staff writers Rochelle Olson and Chao Xiong contributed to this report.

Paul Walsh • 612-673-4482