The case is one of a series raising the national debate about racism and police brutality in the U.S.

As part of a settlement between Mills, the District Attorney's office, and the local U.S. Attorney, Mills agreed to "cooperate fully and truthfully with both the state and federal investigations, including civil rights investigations by the U.S. Justice Department," Mulroy's office said in a statement.

Mills was one of five officers, all Black, to face four federal counts including excessive force and deliberate indifference - both civil rights offenses - and two witness-tampering counts stemming from the beating of Nichols, 29, on Jan. 7. He died in hospital three days later.

They all remain charged with second-degree murder and other counts in Tennessee state court in Shelby County.

Mulroy's office did not say whether Mills would testify against the other defendants as part of the deal. Mills' defense attorney, Blake Ballin, did not respond to requests for comment on whether Mills planned to do so.

Federal and state prosecutors have both recommended a sentence of 15 years in prison for Mills. District Judge Mark Norris of the Western District of Tennessee will decide the sentence at an upcoming hearing, Mulroy's office said.

Police video showed the officers kicked, punched and pepper-sprayed Nichols and struck him with a baton.

At one point, officers held his hands behind his back so another could punch him in the face, while at other times Nichols cried out for his mother, who lived nearby.

Police at the scene alleged he was driving erratically and evaded arrest.

The officers also face a federal civil suit filed by civil rights lawyer Ben Crump on behalf of the family.

The other officers charged in the case are Demetrius Haley, Emmitt Martin III, Justin Smith and Tadarrius Bean.

The Memphis Police Department fired two other officers involved in the incident, suspended three more, and a ninth retired while facing a recommendation for termination, according to city officials.

In addition, four employees of the Memphis Fire Department were disciplined for failing to provide proper medical care for Nichols as he sat against a patrol car with his hands cuffed behind his back.

(Reporting by Julia Harte in New York; Editing by Rod Nickel)