The parents of a Lancaster County man killed in prison claim in a federal lawsuit that state prison staff failed to protect him from his cellmate — a double murderer.

Derric M. Harsh Jr., 23, was found strangled in his cell at State Correctional Institution at Benner Township, on February 27.

Harsh was serving and 8- to 16-year sentence for rape, corruption of minors and unlawful contact with a minor. He pleaded guilty in November 2016.

The federal lawsuit, filed Friday in the U.S. District Court for Eastern Pennsylvania, identifies Harsh's cellmate, Eric Mueller, as the killer.

Mueller is serving two life sentences after pleading guilty to the July 2002 murders of a couple in Philadelphia.

The suit claims prison staff knew Mueller had major mental illnesses and was dangerous.

Earlier in February, the suit said, staff knew that "Mueller was experiencing hallucinations, was feeling homicidal, was distressed that he had lost his single-cell status, was claiming that correctional staff had told him that he would have to kill someone to reclaim single-cell status, and explicitly stated that 'I am on that line to kill someone.'"

Sue McNaughton, a spokeswoman for the Pennsylvania Department of Corrections, said the department doesn't comment on litigation.

"This is a completely senseless death that was avoidable and the family is anxious to make sure that the Department of Corrections takes steps to avoid anything like this happening in the future," Paul Messing, the Philadelphia attorney representing Harsh's family, said Tuesday.

The suit seeks unspecified compensatory and punitive damages, claiming Harsh's Eighth Amendment right against cruel and unusual punishment and his Fourteenth Amendment right to due process were violated.

Mueller has not been charged in Harsh's death.

Centre County District Attorney Bernie Cantorna said he's briefed Harsh's family extensively on the investigation that identifies Mueller as the killer. They understand Mueller is serving two consecutive life sentences, Cantorna said, and don't want charges at this time, but he recognizes their feelings could change.

Messing agreed and said Harsh's family would not be commenting.