A former corrections officer at an Alabama jail has pleaded guilty in the case of a mentally ill man who died of hypothermia after being held naked in a concrete cell for two weeks.

The officer, Joshua Conner Jones, entered into a plea agreement with prosecutors over the treatment of two prisoners at the Walker county jail.

The plea agreement indicated there were five co-conspirators in the mistreatment that led to the 2023 death of prisoner Tony Mitchell, an indication that the investigation is ongoing and more people could be charged in the death.

The plea agreement did not name the inmates, but said it involved a man who died 26 January 2023, after being held in a concrete cell at the jail for two weeks. Mitchell, 33, died after being brought from the jail to a hospital emergency room with a body temperature of 72F (22C), according to a lawsuit filed by his mother.

The plea agreement said that the man “was almost always naked, wet, cold, and covered in feces while lying on the cement floor without a mat or blanket”. By the second week of incarceration, he was “largely listless and mostly unresponsive to questions from officers”, but that the conspirators did not take action to alleviate his suffering.

Prosecutors wrote in the plea agreement that Jones admitted that “collectively we did it. We killed him.”

Jon C Goldfarb, an attorney representing the family in the civil litigation, said “the family is shocked to see in writing what they knew happened to Tony Mitchell”.

Mitchell, who had a history of drug addiction, was arrested on 12 January after a cousin asked authorities to do a welfare check on him because he was rambling about portals to heaven and hell in his home and appeared to be suffering a mental breakdown. The Walker county sheriff’s office posted a photo on its Facebook page, adding that Mitchell, who had his face painted black, “brandished a handgun, and fired at least one shot at deputies” before running into the woods.

Prosecutors wrote in the plea agreement that when Mitchell’s deteriorating condition would be mentioned, the co-conspirators would reply that “‘he gets what he gets since he shot at cops’ or words to that effect”.

Jones agreed to plead guilty to one count of conspiracy to deprive an inmate of their rights related to Mitchell’s death. He also pleaded guilty to a separate rights-deprivation count related to the assault of another inmate.

A defense lawyer for Jones, W Scott Brower, said he could not comment on the agreement. A spokesperson for the US attorney’s office did not immediately return an email seeking comment.