Federal prosecutors recommended a 40-year sentence for a former Memphis police officer who pleaded guilty on Friday to federal civil rights violations in the 2023 fatal beating of Tyre Nichols.

Emmitt Martin is the second former officer to plead guilty in the killing that sparked outrage and renewed calls for police reform. Three former officers still face trial in federal court in September, and two of their former colleagues could testify against them.

Martin entered his change of plea before the US district judge Mark Norris in Memphis under an agreement with prosecutors. He pleaded guilty to excessive force and witness tampering charges. The judge had set a Monday deadline for any plea agreements in the case.

Nichols’s mother, RowVaughn Wells, was in the courtroom. She nodded her head and smiled when the judge accepted Martin’s change of plea.

In a news conference with the civil rights attorney Ben Crump after the hearing, Wells said it was “very emotional” and “bittersweet”. She said the latest plea was a step in the right direction – but that she would not be content until all of the officers are brought to justice.

“Tyre was just coming home. He was just minding his own business,” she said.

Nichols died in a hospital on 10 January 2023, three days after he was kicked, punched and hit with a police baton after a traffic stop. The officers said they pulled Nichols over because he was driving recklessly, but the police chief, Cerelyn “CJ” Davis, has said no evidence was found to support that allegation.

Police video released on 27 January showed the officers beating Nichols as he yelled for his mother about a block away from the home they shared. Video also showed the officers milling about and talking with each other as Nichols struggled with his injuries.

An autopsy report showed Nichols died from blows to the head, and that the manner of death was homicide. The report described brain injuries, cuts and bruises to the head and other parts of the body.

“I will never have my son back. I will never hear his voice again,” Wells told reporters Friday. “They murdered my son for nothing. And until we get justice for all of them, I won’t be content.”

In November, the former Memphis officer Desmond Mills Jr reached a similar deal with federal prosecutors and changed his plea to guilty. Prosecutors recommended a 15-year prison sentence for Mills.

Both Mills and Martin could be called to testify against the final three – Tadarrius Bean, Demetrius Haley and Justin Smith – who remain charged with federal civil rights violations and have pleaded not guilty.

All five former officers accused in Nichols’s death have been charged separately in state court with second-degree murder. That trial is postponed until the federal proceedings are complete.

Nichols was a 29-year-old father from Sacramento, California. He worked at FedEx with his stepfather and enjoyed skateboarding and photography in his spare time. Nichols was Black.

The five accused ex-officers also are Black. They were fired after Nichols’s killing for violating Memphis police department policies.

Martin, who was the second officer to come into contact with Nichols during the 7 January traffic stop, helped Haley to force Nichols from his vehicle, according to documents filed in the case to permanently ban Martin from working in law enforcement in Tennessee.

Nichols ran from Martin and his two partners after they threatened and pepper-sprayed him, but he was apprehended within six minutes. As other officers tried to handcuff Nichols, who was on the ground, Martin kicked him in the upper torso and punched him in the face while two other officers held Nichols’s arms, the documents show.

Martin’s defense attorney, Stephen Ross Johnson, said after the hearing that Martin has accepted responsibility for his actions.

“Emmitt Martin was driven by anger when on January 7 of 2023 he admits that he violated Mr Nichols’s civil rights and used excessive force. He was driven by fear when he later attempted to cover that up – fear of the consequences of what he had done,” Johnson said.

The criminal charges are separate from the US justice department’s “patterns and practices” investigation into how Memphis officers use force and conduct arrests – and whether the department in the majority-Black city engages in racially discriminatory policing.

The justice department also has announced a separate review concerning use of force, de-escalation strategies and specialized units within Memphis police.

Additionally, Nichols’s mother has filed a $550m lawsuit against the city and its police chief.