WARREN, Maine — Maine State Police are calling the Tuesday night death of a convicted murderer incarcerated at the Maine State Prison in Warren a homicide perpetrated by another convicted murderer.

Alan D. Powell, 57, “died last night after being attacked by a second inmate in an exercise area at the Warren prison,” Stephen McCausland, spokesman for the Department of Public Safety, said in a press release issued Wednesday. “Both inmates are convicted murderers.”

Powell was killed at approximately 6:30 p.m. He had served approximately 22 years of a life sentence for murder, Scott Fish, Corrections Department director of special projects, said Wednesday morning.

“Detectives say he was attacked by 42-year-old Guy Hunnewell, who is serving a 40-year sentence for stabbing to death his girlfriend in Fairfield in 1997,” McCausland said.

Guy E. Hunnewell III was sentenced in 1999 in Somerset County Superior Court to 40 years in jail for killing Stephanie Gilliland, 20, of Benton in December 1997 at his Fairfield apartment, according to Bangor Daily News coverage of the trial.

Gilliland, who was Hunnewell’s estranged girlfriend, was stabbed more than 21 times.

Hunnewell was also indicted by the Knox County Superior Court grand jury in March 2003 for allegedly assaulting another inmate with a shank, the BDN archives state.

Powell’s body has been taken to the state medical examiner’s office in Augusta for an autopsy.

Hunnewell remains at the prison, which was visited by detectives Tuesday night who conducted interviews with guards and inmates.

The case likely will be presented at the next session of the Knox County grand jury in July, McCausland said.

“Consistent with the Department policy and the attorney general’s office protocols, the state police and medical examiner were notified,” Fish said. “Both are reviewing the death.”

A law passed in 2011 requires the medical examiner to review any deaths of inmates or people in police custody — including deaths likely the result of natural causes — in order to determine whether an autopsy is needed.

Powell was convicted of murder and gross sexual misconduct in Kennebec County Superior Court in 1991 for the stabbing and strangulation of Martha Daigle, 64, in her Waterville home on June 29, 1989. He was sentenced to life for the murder and a concurrent 20-year sentence for the sexual misconduct.