A teenager has been found guilty of brutally raping and murdering his ninth-grade math teacher.

Philip Chism, now 16, was accused of killing 24-year-old Colleen Ritzer in a bathroom at Danvers High School in Massachusetts on October 22, 2013 - when he was just 14.

Read: Even If Convicted, Teen Accused of Brutally Murdering His Teacher Could Be Freed

Ritzer was raped, strangled, cut and slashed 16 times with a box cutter in a school bathroom before she was sexually assaulted with a tree branch, officials have said. Chism was accused of then using a trash bin to take her body into nearby woods.

On Tuesday, an Essex Superior Court jury found Chism, who was tried as an adult, guilty of first-degree murder, armed robbery and one count of aggravated rape, related to an assault in the bathroom.

But he was found not guilty on another aggravated rape count, which related to an alleged assault in the woods.

For Chism to have been found guilty for this count, the jury would have had to believe she was still alive at the time.

Despite the convictions, Chism will be eligible for parole in 15 years.

His defense team had argued that Chism was in "the throes of mental illness" when he killed Ritzer shortly after he moved to Massachusetts from Clarksville, Tennessee.

But a prosecutor said Chism "knew right from wrong and could choose right from wrong."

Essex County District Attorney Kate MacDougall argued that Chism acted deliberately when he killed his teacher, who was giving Chism extra math help before he killed her.

Read: Philip Chism's Attorney Admits He Killed Teacher, Says Mental Illness Made Him Do 'Terrible Things'

“The defendant strangled Colleen. The defendant used a box cutter he had to cut or slice her throat… He spent 30 minutes in the woods discarding evidence, doing what he wanted, and ultimately discarding Colleen,” MacDougall said, the Boston Globe previously wrote.

Following this trial, Chism is expected to face trial on charges that he attacked and tried to kill a woman working at the Department of Youth Services facility. He has pleaded not guilty to those charges.

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