FARGO (AP) - Prosecutors preparing for the trial of a teenager accused of killing his sister say the defense may try to persuade jurors that he was emotionally disturbed.

"In any murder case, that's a possibility, and I think our job as prosecutors is to consider any possible issues that the defense may raise, and that's just one of them that we've considered," Assistant Cass County State's Attorney Tracy Peters said Friday.

Sergei Carlson 16, of Sun Prairie, Wis., is charged with killing his 16-year-old sister, Whitney, in her south Fargo home last summer. He is to go on trial Monday in Bismarck on charges of murder and a deviate sexual act.

Authorities said Sergei Carlson told police he strangled his sister in her bedroom with his hands, put pillows over her face to muffle her sounds, then had sexual contact with her.

Carlson was adopted from Russia when he was 7 years old and moved with his father to Wisconsin in 2002. Whitney Carlson lived with her mother in Fargo.

Peters said that if the jury agrees that extreme emotional disturbance was a factor in the crimes, the maximum sentence could be reduced to 20 years. A plea agreement rejected twice by Judge John Irby had called for a 30-year sentence.

Defense attorney Mark Beauchene has declined comment on the judge's ruling. In rejecting the plea agreement Thursday, Irby said it did not answer questions on such key issues as rehabilitation.

The trial was moved to Bismarck from Fargo.