Hennepin County Chief Medical Examiner Dr. Andrew Baker's expected testimony Friday about the cause of George Floyd's death could pose challenges for both the state and defense if it mirrors the contents of at least a half-dozen meetings he had with prosecutors about his autopsy results leading up to the murder trial of former Minneapolis police officer Derek Chauvin.

Documents obtained by the Star Tribune show Baker ruled the death a homicide and said Floyd's May 25 arrest placed added stress on his already ailing heart that increased the likelihood of a "bad outcome." But he also expressed skepticism that placing Floyd stomach-down in the street with three officers on top of him would be "any more dangerous than other positions."

In an unusual move that appeared to underscore a growing gap between the state and Baker, prosecutors sought out several outside medical experts, including a forensic pathologist — Dr. Lindsey Thomas — who duplicates Baker's role at trial as a witness on the cause of death. Two additional out-of-state experts testified Thursday.

Dr. Martin Tobin, a Chicago physician who specialized in respiratory and critical care medicine for decades, told jurors that Floyd died due to lack of oxygen that damaged his brain and eventually made his heart stop.

Tobin said Floyd's pre-existing health conditions had nothing to do with his death. Dr. Bill Smock, police surgeon of the Louisville Police Department, also said that Floyd died from a lack of oxygen and not a drug overdose, as argued by the defense.

Baker ruled Floyd's cause of death cardiac arrest. Prosecutors told jurors it was asphyxia.

"They seem to think they want as many experts … because the original report wasn't quite as definitive as they would've liked," said Bradford Colbert, a professor at Mitchell Hamline School of Law.

Chauvin's attorney, Eric Nelson, told jurors in his opening statement last week that prosecutors were dissatisfied with Baker's work. He put them on alert for "several" meetings between Baker and investigators that would factor into their verdict.

The documents show that Baker first met with Hennepin County prosecutors, agents from the state Bureau of Criminal Apprehension (BCA) and the FBI the day after Floyd died. In the videoconference meeting, Baker said he hadn't finished his final autopsy report because he needed to review more evidence.

However, Baker told the group there was no evidence suggesting that Floyd died of asphyxiation, according to records.

Baker said Floyd had several external injuries, including a cut to his lip and bruising on his left shoulder and face. He also noted the presence of pre-existing health conditions. He said he had not watched video of Floyd's arrest to avoid biasing the autopsy, the records said.

Baker met with prosecutors in person the next day.

On May 31, Baker again videoconferenced with assistant Hennepin County attorneys Patrick Lofton and Amy Sweasy and walked them through Floyd's toxicology results, which showed the presence of the potent synthetic opioid fentanyl and methamphetamine in his blood stream, according to the documents.

He told them if Floyd had been found dead at home "and there were no other contributing factors" he would have concluded that the death was from an overdose, according to a summary.

The next day the Hennepin County Medical Examiner's Office publicly ruled Floyd's cause of death "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." It also listed heart disease, thickening of the artery walls and fentanyl and methamphetamine as "other significant factors." The manner of death was ruled a homicide, an act that occurs at the hands of another person.

Over the ensuing months, Baker met several times with the Minnesota Attorney General's Office, which took over lead prosecution of the case. He was also interviewed by the FBI, which is conducting a separate federal investigation into Chauvin. On Sept. 1 he met via videoconference with federal prosecutors, the FBI and the BCA. Baker's attorney accompanied him at times.

According to court documents, in a meeting with the FBI, Baker said he could not say whether Floyd would have lived "but for" the officers' use of force.

"Baker did not believe that the prone position was any more dangerous than other positions based on an article or journal he had read," said a summary of the meeting.

However, he also said in the same meeting that the exertion of the arrest was "more than Floyd could tolerate" and likely increased his heart rate and adrenaline levels while placing greater demand on his breathing.

Prosecutors, who have focused heavily on the dangers of prone restraint, will also call Thomas to testify that Floyd died of asphyxiation.

Nelson has argued that Floyd succumbed to a drug overdose and pre-existing health issues. Chauvin is charged with second-degree murder, third-degree murder and second-degree manslaughter.

Longtime defense attorney Joe Friedberg said he has never known the county to go outside its own Medical Examiner's Office for opinions.

"I think they're not happy with the conclusions of Dr. Baker, so they are seeking to buttress the evidence as it relates to cause of death," Friedberg said, adding that such a strategy could have broad implications for other cases. "You've gotta remember, there's a whole lot of people in prison, based solely on Dr. Baker's cause-of-death testimony, on behalf of Hennepin [County] and other towns."

Medical examiners operate independently of police and prosecutors and do not determine legal culpability in a death.

As Chauvin knelt on Floyd's neck, officer J. Alexander Kueng knelt on his back and officer Thomas Lane knelt on and held onto his legs. Officer Tou Thao kept bystanders at bay. Kueng, Lane and Thao are scheduled to be tried Aug. 23 for aiding and abetting second-degree murder and manslaughter. All four defendants, who were fired, are out on bond.

Records show that another potential state's witness told prosecutors in a March 22 videoconference that the medical examiner's "lack of pathological findings suggest asphyxia played a role in Floyd's death."

Other records obtained by the Star Tribune show that the U.S. Department of Justice requested an independent evaluation of Baker's autopsy findings by the Office of the Armed Services Medical Examiner. That office agreed with Baker's conclusions after reviewing his report and other evidence.

Dr. Edward Willey, a former medical examiner who works as a forensic pathology consultant in Florida, said government agencies don't typically spend a lot of money on outside experts, but prosecutors in the Chauvin trial likely sought outside help due to the case's complexity. Cases of potential asphyxiation "always present some problems because they're not as clear cut as many others," he said. "When you have a bullet wound, that's much more definitive."

Prosecutors only have to show that Chauvin's actions were a "substantial causal factor" in Floyd's death even if other issues contributed to it, according to Minnesota guidelines for jury instructions.