Former police officer Derek Chauvin has filed a new motion attempting to overturn his federal civil rights conviction in the murder of George Floyd.

The motion to vacate comes more than three years after Floyd's murder in May 2020, when Chauvin knelt on the Black man's neck for more than nine minutes. The world responded to the killing with outrage, riots and protests by the thousands.

In July 2022, a federal judge sentenced Chauvin to more than 20 years in prison for violating the civil rights of Floyd and a Black Minneapolis teen for excessive use of force during an encounter in 2017.

Chauvin alleges in the motion that his attorney at the time, Eric Nelson, didn't tell him that Dr. William Schaetzel, a Kansas-based pathologist, offered to testify that Chauvin was not responsible for Floyd's death.

The former officer said he would not have pleaded guilty to the federal charges if he had known about the offer to testify.

Floyd's cause of death was a pivotal issue at trial. The Hennepin County Medical Examiner ruled Floyd's death to be a homicide after he had cardiac arrest likely caused by the restraint. Doctors hired by Floyd's family said they believe he died of asphyxia.

Schaetzel said he reviewed the autopsy and believes that it was not asphyxia but rather a rare tumor called paraganglioma that killed Floyd.

Chauvin wrote in the motion that Schaetzel contacted Nelson in April 2021 to say he was willing to testify. Chauvin claims that Nelson never responded to that offer or informed Chauvin.

Nelson declined to comment on the motion in an email Wednesday.

Chauvin's defense argued at trial that Floyd died of natural causes related to his drug use.

After reviewing the motion, Minneapolis attorney Joe Tamburino said it would be a "long shot" for Chauvin to be awarded a new trial through this motion. He said it's "extremely, extremely rare" for a judge to reverse a conviction based on ineffective counsel.

Tamburino said he thinks it is possible for Chauvin to be given a hearing regarding his motion.

Even if Chauvin had his federal conviction vacated, he would still be required to serve his state conviction of more than 22 years in prison.