An Auburn man convicted in the stabbing death of a Bates College student pleaded guilty to aggravated assault for stabbing an inmate at Maine State Prison, where he is serving the murder sentence.

Brandon Thongsavanh, 24, was sentenced to 15 months at the Warren prison, where he has been in custody since his second conviction for the 2002 murder of Bates College senior Morgan McDuffee. Thongsavanh’s, whose first conviction was overturned on appeal, is serving a 58-year sentence imposed after a second conviction. The 15-month term will be served in addition.

Thongsavanh was indicted last summer in the prison stabbing on charges stemming from an incident involving another inmate.

The victim suffered stab wounds to his chest and legs, according to Knox County District Attorney Geoffrey Rushlau, who prosecuted the case. Thongsavanh was captured on video by surveillance cameras following Charles Limanni into his cell and leaving alone shortly before the stabbing was reported. Another inmate reportedly served as lookout. It wasn’t clear what the origin of the dispute was, Rushlau said.

The weapon, believed to be a knifelike shank, was never recovered, Rushlau said. Limanni has since been released and is believed to be living in Cumberland County.

Rushlau said prosecution would have been problematic had the case gone to trial.

“Cases like this are inherently difficult,” he said. Prison inmates are generally uncooperative and don’t like to testify for the state, he said.

A grand jury handed up an indictment a year ago.

Rushlau had pressed for a two-year sentence. Thongsavanh successfully argued for less.

His murder sentence will be set aside while he serves the nonconcurrent 15-month term, then will resume, Rushlau said. He arranged for the interruption of the first sentence in order to send a message to prison inmates that their actions while in state custody carry real consequences.

Thongsavanh was convicted of McDuffee’s murder in connection with a street fight on Main Street in Lewiston in which a group of Bates students clashed with a group of local teens. McDuffee was stabbed five times, once in the heart.

For about a year, while awaiting retrial, Thongsavanh was shipped to Arizona in a prisoner swap. Jail officials in Maine said that while he was being held in Arizona, Thongsavanh was an enforcer for a group of inmates and was involved in an incident during which another prisoner’s throat was slashed.