A former Memphis police officer pleaded guilty Thursday to federal charges of violating the civil rights of Tyre Nichols, a Black man who died after a beating by police in January, authorities said.

Desmond Mills Jr., 33, switched his not-guilty plea in an agreement with prosecutors, becoming the first person convicted in a case that reignited nationwide outrage around police misconduct and excessive force against Black people. Mills faces up to 15 years in prison and is scheduled to be sentenced May 22, authorities said.

After a federal court hearing, Shelby County District Attorney Steven Mulroy said Mills had also agreed to plead guilty to aspects of state felony charges - second-degree murder, aggravated assault and aggravated kidnapping - filed against him and four other former officers three weeks after the Nichols beating in January. The plea in the state case will come after the federal sentencing, Mulroy said.

Mulroy said his office, in an agreement that includes federal prosecutors, Mills's attorneys and the Nichols family, accepted that Mills will serve any sentence in federal prison.

"I join Tyre's family in saying this is a fair result, given Mr. Mills' level of involvement, and his willingness to cooperate with us," Mulroy said in a statement. "His cooperation will help us bring to justice all those criminally responsible while also identifying needs for systemic reform within the police department."

Nichols, 29, was pummeled by several officers for about three minutes on Jan. 7. The beating was captured on surveillance video and police body cameras. He died of his injuries three days later.

In September, federal authorities indicted Mills and four other former officers - Emmitt Martin III, Tadarrius Bean, Demetrius Haley and Justin Smith - for using excessive force and violating Nichols's rights, as well as conspiring to make false statements to supervisors while reporting what happened and obstructing justice during subsequent investigations. All of the ex-officers are Black.

On Thursday, Mills pleaded guilty to two of the four federal counts, for using excessive force and making false statements, authorities said; prosecutors said they will drop the other two counts when Mills is sentenced.

Authorities said Mills admitted in court to unjustifiably striking Nichols with a police baton, failing to intervene while other officers used excessive force, neglecting to render assistance to Nichols for his injuries and conspiring to make false statements about the incident to his superiors.

Martin, Bean, Haley and Smith have pleaded not guilty in the federal case with a trial set for May 6. They also have pleaded not guilty to the Tennessee state charges.

The federal and state prosecutions are only part of the fallout from Nichols's death.

The Memphis Police Department is undergoing a Justice Department civil investigation over its culture, training and policies. The police department has disbanded the specialized Scorpion unit, which focused on drug cases, whose officers were involved in the Nichols beating.

The Nichols family has filed a federal civil rights lawsuit against the city, the police department, the police chief, fire department officials and the five former officers involved in the beating.

In August, Mulroy threw out charges in 30 cases investigated by the former police officers charged in Nichols's death and reduced the charges in a dozen other cases. He cited concerns over the former officers' credibility.

"Desmond Mills' plea today is entirely consistent with our allegations in the civil lawsuit against the City of Memphis," Ben Crump and Antonio Romanucci, attorneys for the Nichols family, said in a statement. "We stand strong in our belief that these officers, including Mills, acted at the direction of a policy that not only violated civil rights of innocent civilians but which caused needless pain to many."