A Massachusetts teenager has been convicted of raping and killing his high school maths teacher.Jurors returned the guilty verdict against Philip Chism on Tuesday over the 2013 murder of 24-year-old Colleen Ritzer.

Colleen Ritzer

Chism was 14 when authorities said he followed Ms Ritzer into the girls’ bathroom after school, where he strangled her, cut her throat with a box cutter, raped and killed her.

The popular Danvers High School teacher’s body was later found in a wooded area near the school.

Chism’s lawyers admitted the teen killed Ms Ritzer, but asked the jury to find him not guilty by reason of insanity.

Defence attorney Denise Regan said at trial that Chism suffers from a psychotic disorder that triggers hallucinations, depression and anxiety.

She claimed the teen was in the midst of a psychotic episode when he killed his teacher.

Jurors were shown chilling surveillance video that showed Chism putting on a pair of gloves as he followed Ms Ritzer into the bathroom on 22 October 2013.

Minutes later, he was seen emerging from the bathroom carrying the black pants the teacher had been wearing.

Additional footage showed Chism pulling a recycling barrel through the school and outside. The recycling bin was found near Ms Ritzer’s body.

Chism was found hours later walking down a road carrying Ms Ritzer’s credit cards, her underwear and a box cutter covered in blood.

Now 16, he was tried as an adult.

The trial was brought to a brief halt at one point when he refused to leave his holding cell following a break in testimony.

Ms Regan told the judge he was “shaking, twitching and mumbling” and told her he was “about to explode”.

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