A high school student is charged with murder and aggravated rape in Salem, Mass. after allegedly karate chopping his math teacher and cutting her throat with a box cutter over a “trigger word.”

Philip Chism, 15, is accused of killing Colleen Ritzer, 24, on Oct. 22, 2013. Chism, 14 at the time, explained to police that he followed Ritzer into a school bathroom wearing a hood and gloves. He then used a karate chop to knock her out and sliced her neck twice with a box cutter, ABC-affiliate WCVB reports.

Police Detective Lt. Norman Zuk testified for prosecutors Friday that Chism said he was set off by a “trigger word,” but Zuk did not say what the word was. Trigger words are words or phrases that cause a strong emotional reaction or reliving of a traumatic event.

Ritzer was found in the woods near the school with a note saying “I hate you all.” She was naked from the waist down, and police say she was sexually assaulted with a stick. Surveillance video shows Chism following Ritzer into the bathroom, leaving alone and later dragging a recycling barrel through the school.

Police found Chism around midnight on Oct. 23, and Chism reportedly said a women who could not be helped was “buried in the woods.” Officers testified that they found a bloody box cutter, Ritzer’s ID and credit cards and a pair of women’s underwear on Chism, WCVB states.

Lawyers for Chism are attempting to get his confession thrown out because they say police coerced Diana, his mother, into waiving his rights. Prosecutors say Diana never invoked their right to a lawyer.

Chism has also been charged with attempted murder and kidnapping after attacking a female staff member at Dorchester Department of Youth Services where he is being held before his trial, CNN reports. He reportedly followed her into a bathroom, choked her and repeatedly punched her in the face.

Chism is being tried as an adult and has pleaded not guilty for Ritzer’s murder.

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