A convicted killer being sued by the mother of a teenage girl he stabbed to death at their Connecticut high school after she rejected his prom invitation plans to have a psychiatrist testify he was mentally ill and couldn't control himself, if the civil case goes to trial.

Atlanta psychiatrist Peter Ash would testify about Christopher Plaskon's mental illness if the case isn't settled or dismissed, according to a court document filed by Plaskon's lawyer earlier this month that disclosed Ash as an expert witness for the defense.

The document did not identify the illness, but Plaskon's parents told authorities that he had possible depression, suicidal tendencies, self-mutilating behavior and mood swings. Other court documents said Plaskon heard voices in his head that made him kill the girl.

Prosecutors said Plaskon fatally stabbed 16-year-old classmate Maren Sanchez in a hallway at Jonathan Law High School in Milford on April 25, 2014. Plaskon told a friend he was upset that Sanchez had rejected his invitation to the junior prom, according to court documents. The prom was to be held hours later on the day of the killing but was postponed.

Plaskon, now 20, pleaded no contest to murder last year and was sentenced to 25 years in prison. His criminal case lawyer had considered an insanity defense before he took the plea deal, saying he showed signs of psychosis.

Sanchez's mother, Donna Cimarelli-Sanchez, filed a wrongful death lawsuit last year against Plaskon, his parents and the city of Milford. The lawsuit alleges Plaskon's parents and school officials could have prevented the killing by taking appropriate steps to address his mental illness. Plaskon's parents and school officials deny wrongdoing.

Ash, the psychiatrist, would testify if the lawsuit goes to trial.

"Dr. Ash will testify that at the time of the incident ... this defendant, as a result of mental disease, lacked substantial capacity to appreciate the wrongfulness of his conduct," Plaskon's lawyer, Peter Ponziani, wrote in the expert witness disclosure. "He will further opine that at the time of the incident, as a result of mental illness, this defendant lacked substantial capacity to control his conduct within the requirements of the law."

Ponziani did not return messages seeking comment. He said in the court document that Ash had evaluated Plaskon, reviewed his medical records and interviewed his doctors and relatives.

David Golub, a lawyer for Cimarelli-Sanchez, did not respond to a message seeking comment about Ash on Friday.

Sanchez was a member of the National Honor Society and was active in drama and other school activities. She had posted on Facebook a photograph of herself wearing a prom dress and was looking forward to attending with a new boyfriend.

The attack happened in a first-floor hallway at about 7:15 a.m. Students described an emotional scene where people were crying as police and paramedics swarmed the school.

A witness tried to pull Plaskon off Sanchez during the attack, and another saw Plaskon discard a bloody knife, according to an arrest warrant affidavit. Plaskon was taken to the principal's office in bloody clothing and told police, "I did it. Just arrest me," according to the affidavit.

Staff members and paramedics performed life-saving measures on Sanchez, but she was pronounced dead at a hospital shortly afterward. The medical examiner's office said she was stabbed in the torso and neck.