EUGENE, Oregon (AP) -- Kip Kinkel, the teen-ager accused of murdering his parents and then killing two students in a shooting rampage at his Springfield high school, will use a mental illness defense that could someday result in his freedom.

Kinkel, 16, goes on trial as an adult on Sept. 27. He will not face the death penalty because he was only 15 when the crime took place.

According to court papers released Monday, his lawyers will present three legal theories.

They will argue that Kinkel suffers a mental disease or defect that prevented him from understanding his conduct was criminal, or prevented him from obeying the law. If that succeeds, he would be found guilty but insane and would be confined for treatment until a court decides he can be released.

The other defense avenues are partial responsibility, in which Kinkel's lawyers would claim he couldn't form the intent to commit the crimes because of mental disease or defect, and extreme emotional disturbance. They could lead to manslaughter convictions rather than murder.

District Attorney Doug Harcleroad would not comment because of a court order barring the two sides from discussing the case.

However, in court papers, prosecutors have said Kinkel was alert and responsive to questions after the shootings. They noted that he has no history of mental disease and did not seem to be hearing voices, suffering hallucinations or exhibiting other symptoms of a breakdown.

Kinkel is accused of fatally shooting his parents, Bill and Faith Kinkel, on May 20, 1998, then going the next day to Thurston High School, killing two students and wounding 22 others.