SALEM, Mass. (CBS/AP) — A Massachusetts teenager raped and killed his high school math teacher but was driven by severe mental illness, his defense attorney told jurors Monday.

Philip Chism, 16, is charged with murder in the October 2013 slaying of Colleen Ritzer. The body of the 24-year-old teacher at Danvers High School, north of Boston, was found near the school. She had been raped and her throat had been slit.

Chism was 14 at the time. He has pleaded not guilty and is being tried as an adult in Essex Superior Court in Salem.

“Philip Chism killed Colleen Ritzer,” and did “unspeakable things to her body,” defense attorney Denise Regan said in her opening statement to jurors.

But Chism, who had moved to Massachusetts from Clarksville, Tennessee, should not be held criminally responsible, his attorney said. The teenager was “severely mentally ill” and had suffered from a psychotic disorder since the age of 10, Regan said.

Jury selection had been suspended several weeks while the judge considered Chism’s mental competency before finding the teenager competent to understand the court proceedings and assist with his defense.

Prosecutor Kate MacDougall said in her opening statement that Chism went to school on the day Ritzer died with a “terrible purpose”: to kill his teacher.

MacDougall told jurors about Ritzer’s final day and how she arrived at school just hours before being murdered. Ritzer brought a backpack, purple lunch tote and “the knowledge that she was doing the job she had always dreamed of: teaching math,” the prosecutor said.

She said Chism came to school with a backpack containing a box cutter, gloves and a mask. She said Chism followed Colleen Ritzer into the girls’ bathroom after school, strangled her, cut her throat with the box cutter, raped and killed her.

One of the first witnesses to testify was a police officer who spotted Chism walking along a highway in the neighboring town of Topsfield and brought him to a police station.

Officer Neal Hovey said he found a bloody box cutter and a woman’s wallet in Chism’s backpack. When he asked Chism whose blood it was, he replied, “It’s the girl’s,” Hovey testified.

WBZ NewsRadio 1030’s Ben Parker reports

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