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What killed George Floyd? Chauvin trial prosecution is having trouble answering.

There is still a significant cause-of-death battle to be fought in the coming trial days.

As the nation watches the trial of former Minneapolis police Officer Derek Chauvin in connection with the death of George Floyd for a third week, we've seen medical expert after medical expert testify about their opinions and conclusions regarding the cause of Floyd's death.

Curiously, the first few of those opinions we heard last week were offered by medical professionals who did not perform the autopsy on Floyd's body.

Curiously, the first few of those opinions we heard last week were offered by medical professionals who did not perform the autopsy on Floyd's body. That came later, at the end of a string of expert witnesses, when we finally got to hear from the forensic pathologist who actually performed the autopsy, Hennepin County Chief Medical Examiner Andrew Baker.

As Baker began his testimony, it became apparent why the prosecutors buried him deep in the batting order: His opinions and conclusions are problematic for the prosecution.

Hennepin County Medical Examiner Dr. Andrew Baker testifies during the trial of former Minneapolis police officer Derek Chauvin on April 9, 2021.Jane Rosenberg / Reuters

On Monday, the prosecution tried to redirect the jury's attention back to more favorable medical opinions by calling an expert cardiologist, Dr. Jonathan Rich. Like Dr. Martin Tobin and Dr. Lindsey Thomas, a retired medical examiner, last week, Rich told the jury that Floyd died of asphyxiation as a result of low oxygen caused by police restraint.

In essence, the prosecution sandwiched Baker's weaker opinion between the other medical experts' stronger opinions. But the question remains: How much damage did Baker do to the prosecution's case?

As a homicide prosecutor for more than two decades, I have called countless medical examiners to the stand, including Dr. David Fowler, who has been named as one of the expert witnesses who will testify in Chauvin's defense. In my murder trials, there were times when the cause of death was so particularly vexing that I had to call multiple medical examiners.

On those occasions, I would first call the medical examiner who performed the autopsy and then call a second forensic pathologist to supplement, shore up or explain the testimony of the first medical examiner, particularly when the second medical examiner had a specialty or expertise that the primary medical examiner lacked. In the Chauvin case, this more conventional approach was flipped on its head.

Before taking on Baker's testimony, let's review the medical professionals who served as the opening acts. I, for one, was transfixed by Tobin, the expert pulmonologist. For a few hours, he served as the viewing audience's learned professor, taking us all through the last moments of Floyd's life, culminating in his identifying, in the video of the incident, the precise "moment the life goes out of his body."

Importantly, Tobin provided what I would call the 1-2-3 of George Floyd's death: 1. Floyd suffered cardiopulmonary arrest (his heart and respiration stopped). 2. The cardiopulmonary arrest was a result of a deprivation of oxygen. 3. The deprivation of oxygen was caused by the police restraint, including positioning him facedown on the asphalt, cuffing his hands behind his back and placing pressure on his torso and neck.

Baker used language that I found unusual for a forensic pathologist.

We then met Thomas, the retired Hennepin County medical examiner who trained and supervised Baker. Thomas was asked about Baker's findings that Floyd's death was caused by "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." Specifically, prosecutors asked Thomas what those findings mean to her, to which she replied, "What it means to me is that the activities of the law enforcement officers resulted in Mr. Floyd's death."

Thomas put some icing on the culpability cake by adding, "There's no evidence to suggest he would have died that night except for the interaction with law enforcement." As I watched Thomas testify, I could not shake the feeling that some dark cause-of-death clouds were gathering over the prosecution's case given that prosecutors were asking her what Baker's findings meant to her, before we ever heard the primary source, Baker himself, tell the jury what his findings meant to him.

I fully expected that Baker would elaborate on some of his findings in ways that were consistent with Tobin and Thomas. For example, I awaited his explanation of how he could find "no life-threatening injuries" but still rule the death a "homicide."

I looked forward to his discussion of how someone could suffer asphyxial death, with a knee pressed on his neck for more than 9 minutes, yet have no injuries to the neck either externally or internally. I fully expected that Baker would say Floyd died from a deprivation of oxygen.

But when Baker finally hit the stand, it became obvious that the prosecution had "softened the ground" for the jury with the opinions and conclusions of the other expert witnesses. Baker seemed far more inclined to believe that Floyd's alleged drug use, as evidenced by the toxicology tests showing that he had fentanyl, methamphetamines and other drugs in his system, as well as underlying heart disease, were contributing factors to his death.

Critically, unlike the other medical expert witnesses, Baker did not say that a lack of oxygen or asphyxiation caused Floyd's death.

Baker used language that I found unusual for a forensic pathologist, saying, for example, "The law enforcement subdual, restraint and the neck compression was just more than Mr. Floyd could take by virtue of those heart conditions." First, language like "more than his heart can take" is imprecise coming from a medical professional and will give the defense an opening to argue that Floyd's heart condition was a significant cause of death, as opposed to the improper, prolonged police restraint.

I thought Baker rallied a bit when he testified that "the other significant conditions," meaning heart disease and drug use, "were things that played a role in the death but didn't directly cause the death."

However, he then offered this curious formulation: "So, for example, Mr. Floyd's use of fentanyl did not cause the ... neck restraint, his heart disease did not cause the ... neck restraining." Huh? The relevant question is not whether his heart disease or his drug use caused the neck restraint. The question is whether the neck restraint (and other improper police application of force) caused Floyd's death.

On cross-examination, defense attorney Eric Nelson scored significant points on the cause-of-death front when he asked Baker whether "the heart disease, as well as the history of hypertension and the ... drugs that were in his system, played a role in Mr. Floyd's death." Baker responded, "In my opinion, yes."

Critically, unlike the other medical expert witnesses, Baker did not say that a lack of oxygen or asphyxiation caused Floyd's death. It looked to me like the prosecutor and the witness were equally uncomfortable in the exchange. Indeed, the prosecution seemed anxious to move through Baker's testimony as quickly as possible.

We may learn the answer to the question of how damaging Baker's testimony was to the prosecution when Fowler, the defendant's expert forensic pathologist, testifies. Fowler was the chief medical examiner for Maryland for nearly two decades. He is an accomplished forensic pathologist and a strong testifying witness. I worked multiple cases with him in my time as a homicide prosecutor at the U.S. attorney's office for the District of Columbia, including a case involving a death that occurred during the use of force (though the case involved civilians, not police officers).

I expect that Fowler will actually embrace the opinions and the testimony of Baker. He is likely to say that just as Baker did not conclude that asphyxiation was the cause of death, he also concludes that this was not an asphyxial death. I suspect Fowler will focus on the heart infirmities and the drugs in his system as significant contributors to Floyd's death.

In the meantime, be on the lookout for much discussion of how the body will produce an adrenaline surge during police encounters, particularly when restraint is applied, and how an extreme adrenaline surge can affect the heart, respiration, blood flow and other bodily functions and processes.

All of this is to say that in the wake of Baker's testimony, there is still a significant cause-of-death battle to be fought in the coming trial days.