FARGO (AP) - A teenager sentenced to life in prison for killing his sister has been ordered to serve five years at the same time for assaulting a detention center officer while trying to escape.

Sergei Carlson, 16, pleaded guilty Wednesday in the Sept. 25 incident assault and escape attempt.

Authorities said he struck a detention center officer in the head with trim from a bookcase before being tackled by another officer at the juvenile detention center in Fargo.

Assistant Cass County State's Attorney Tracy Peters said authorities believe others were involved in the escape attempt, but she was not certain of potential charges.

Carlson was sentenced to life in prison with the possibility of parole last month for killing his sister Whitney and performing a deviate sexual act in July 2007. Authorities said he strangled his sister in her Fargo bedroom then had sex with her.

Sergei Carlson, who was adopted from Russia when he was 7, had been living in Sun Prairie, Wis., with his father. Whitney lived with her mother and sisters in Fargo.

Peters said Carlson will begin serving his life sentence, which includes the possibility of parole, at the state Youth Correctional Center in Mandan.

"He will stay at Youth Correctional Center until he graduates from high school and then make transition to state penitentiary," Peters said.

Officials at the Mandan youth facility are preparing for Carlson's arrival knowing about the escape attempt, she said.

"They're fully aware of everything that's happened. They've been involved in all the proceedings on this new charge," she said.

Prosecutors were not opposed to having Carlson serve the sentence on the escape attempt at the same time as his murder sentence, she said.

"When he's serving a life sentence, there wasn't much point in tacking on any additional time," she said.