FARGO - A judge has ordered a mental evaluation for a Wisconsin teenager preparing to go to trial next month for the death of his sister.

Prosecutors asked for the evaluation of Sergei Carlson, who is charged with murder and a deviate sexual act in the death of his 16-year-old sister, Whitney. His trial is set for Sept. 15 in Bismarck.

Assistant Cass County State's Attorney Tracy Peters said Tuesday the exam is meant to determine whether Sergei Carlson, 16, was suffering from extreme emotional disturbance at the time of his sister's death on July 2007.

"Issues have been raised by the defense and we believe an evaluation is appropriate," Peters said afterward. She would not elaborate.

Defense attorney Mark Beauchene did not object to the evaluation. He left the courthouse without commenting.

East Central District Judge John Irby, who earlier rejected a plea agreement in the case, denied a request from prosecutors to delay the trial so they could have more time to review results of the evaluation.

"This trial will be as scheduled," Irby said.

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.