ROCKLAND — A 43-year-old Maine State Prison inmate will spend the rest of his life behind bars after he was convicted Tuesday of beating a fellow prisoner to death with an electric guitar last summer.

Guy E. Hunnewell pleaded no contest Tuesday in Knox County Superior Court to both the murder charge and an unrelated aggravated assault charge. Justice Jeffrey Hjelm then sentenced Hunnewell to life for the murder.

The no-contest pleas result in convictions but allow Hunnewell to challenge his liability in any subsequent civil trial.

Hjelm said a life sentence was required under state law and state supreme court rulings because the murder occurred within a prison and because Hunnewell was already serving time for murder. The judge called the June 2013 attack that led to the death of 57-year-old Alan D. Powell unprovoked and savage.

Assistant Attorney General Leane Zainea said that if the case had gone to trial, witnesses would have testified that Hunnewell and Powell were provided guitars and allowed to play in the recreation area of the special management unit of the Maine State Prison. A third prisoner who was in the recreation area soon returned and appeared pale, which caused a guard to rush out and find Powell lying facedown in a pool of blood while Hunnewell stood over him, striking him.

Another guard in another section of the prison witnessed the attack through a window and said that Powell was sitting in a chair and that Hunnewell used the guitar like a club and struck the victim multiple times, including as Powell was face down on the ground.

The third inmate told prison authorities that there was no hostility between Hunnewell and Powell before the attack, Zainea said. Hunnewell later told a prison investigator, however, that he believed Powell was “trying to set him up.”

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. In 2004, Hunnewell was convicted of aggravated assault and possession of prison contraband.

Defense attorney Philip Cohen said Hunnewell chose to plead no contest to the new charges against the advice of his attorneys. Cohen said he believed there would have been a good case to seek an acquittal based on a lack of criminal responsibility and Hunnewell’s abnormal state of mind.

Dr. Andrew Wisch, a state forensic psychologist, told the judge Tuesday that Hunnewell was competent to stand trial and to proceed with the hearing.

Hunnewell declined to challenge his guilt or the life sentence recommended by the attorney general’s office.

Hjelm also ordered Hunnewell to pay restitution of $2,470 to the state’s victim compensation fund for funeral expenses it paid in connection to Powell’s death.

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 initially pleaded not guilty to the Powell murder charge in August and not guilty by reason of insanity.

Hunnewell was also charged with aggravated assault in connection to a separate incident in which he is accused of striking fellow prisoner James E. Brensinger, 29, with a shovel in January 2013. Zainea said evidence would have shown that Hunnewell and Brensinger were given shovels to remove wax buildup outside their cells when Hunnewell began striking Brensinger. Hjelm gave him an additional five years for that offense.

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.

In the past five years, four prisoners in Warren have been killed. In one case, a prisoner was acquitted in connection to a beating death, and in the second, the defendant pleaded to the lesser offense of aggravated assault.

The latest involved Richard Stahursky, 35, who is charged with the Feb. 28 murder of Micah Boland. Police say Boland was stabbed 87 times and beaten in his cell at the Maine State Prison. Stahursky had two homemade knives in his possession at the time of the killing.