ROCKLAND, Maine — A Maine State Prison inmate accused of beating another prisoner to death with an electric guitar last summer is scheduled to change his plea next week.

The plea hearing for Guy E. Hunnewell, 43, is scheduled for Tuesday.

Hunnewell was indicted in July 2013 for murder in connection with the June 25, 2013, beating death of fellow prisoner Alan D. Powell. Powell, 57, died from blunt force trauma of the head, according to the Maine medical examiner’s office.

Hunnewell pleaded not guilty in August and not guilty by reason of insanity.

Hunnewell is serving a 40-year prison sentence for killing his estranged girlfriend Stephanie Gilliland, 20, of Benton at his Fairfield apartment in December 1997. She suffered more than 21 stab wounds in that fatal attack.

Powell had served about 22 years of a life sentence for the June 1989 stabbing and strangulation death of Martha Daigle, 64, in her Waterville home. The victim was sexually assaulted.

Hunnewell is also charged with aggravated assault in connection to a separate incident when he is accused of striking fellow prisoner James E. Brensinger, 29, with a shovel in January 2013.

Brensinger backed out of a plea agreement in November on a charge that he sexually assaulted a health care worker at the prison, claiming the injury he suffered in the assault affected his thinking.

Brensinger filed a federal lawsuit in January against the state and Hunnewell because of that attack. Brensinger stated in court papers that $60,000 would make him happy.