A Massachusetts teenager charged with killing and raping his high school math teacher "did unspeakable things to her body" and was "severely mentally ill," his lawyer told jurors during opening statements on Monday.

Denise Regan told jurors that Philip Chism killed Colleen Ritzer at Danvers High School on Oct. 22, 2013, but that the teenager was suffering from a psychotic disorder. Regan said Chism, who had moved to Massachusetts from Tennessee, was "severely mentally ill," and had suffered from a psychotic disorder since the age of 10.

Now 16, Chism is being tried as an adult in Essex Superior Court in Salem. He was 14 when Ritzer's body was found in the woods near the high school, her throat slit. He has pleaded not guilty by reason of mental defect. 

Jury selection was 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.

A prosecutor in the case told jurors that Chism went to school on the day Ritzer died with a "terrible purpose." During opening statements, the prosecutor described Chism's movements, saying the teen came to school with a backpack containing a box cutter, gloves and a mask, followed Ritzer into the girls' bathroom after school, then strangled her, cut her throat with the box cutter, raped and killed her.

A police officer who found a bloody box cutter inside Chism's backpack testified that when he asked the teen whose blood it was, the teen replied, "It's the girl's."

Topsfield Officer Neal Hovey was one of the first witnesses in Chism's trial. 

Hovey said he spotted Chism walking along a highway and brought him to a police station where investigators found a bloody box cutter in a woman's wallet inside Chism's backpack.