MINNEAPOLIS (AP) — Former Minneapolis police Officer Derek Chauvin is making another attempt to overturn his federal civil rights conviction in the 2020 murder of George Floyd, saying new evidence shows that he didn’t cause Floyd’s death.

In a motion filed in federal court Monday, Chauvin said he never would have pleaded guilty to the charge in 2021 if he had known about the theories of a Kansas pathologist with whom he began corresponding in February. Chauvin is asking the judge who presided over his trial to throw out his conviction and order a new trial, or at least an evidentiary hearing.

Floyd, who was Black, died on May 25, 2020, after Chauvin, who is white, kneeled on his neck for 9 1/2 minutes on the street outside a convenience store where Floyd tried to pass a counterfeit $20 bill. A bystander video captured Floyd’s fading cries of “I can’t breathe.” Floyd’s death touched off protests worldwide, some of which turned violent, and forced a national reckoning with police brutality and racism.

Chauvin, who is serving a 21-year sentence at a federal prison in Arizona, filed the request without a lawyer. He says Dr. William Schaetzel, of Topeka, Kansas, told him that he believes Floyd died not from asphyxia from Chauvin’s actions, but from complications of a rare tumor called a paraganglioma that can cause a fatal surge of adrenaline. The pathologist did not examine Floyd’s body but reviewed autopsy reports.

“I can’t go to my grave with what I know,” Schaetzel told The Associated Press by phone on Monday, explaining why he reached out to Chauvin. He went on to say, “I just want the truth.”

Chauvin further alleges that Schaetzel reached out to his trial attorney, Eric Nelson, in 2021, as well as the judge and prosecution in his state-court murder trial, but that Nelson never told him about the pathologist or his ideas. He also alleges that Nelson failed to challenge the constitutionality of the federal charge.

But Chauvin claims in his motion that no jury would have convicted him if it had heard the pathologist’s evidence

Nelson declined to comment Monday.

When Chauvin pleaded guilty to the federal charge in December 2021, he waived his rights to appeal except on the basis of a claim of ineffective counsel.

A federal appeals court has rejected Chauvin’s requests for a rehearing twice. He’s still waiting for the U.S. Supreme Court to decide whether it will hear his appeal of his state court murder conviction.

Three other former officers who were at the scene received lesser state and federal sentences for their roles in Floyd’s death.

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This story has been corrected to show that the doctor is a pathologist, not a forensic pathologist.