A Massachusetts prosecutor on Monday described a 16-year-old former high school student charged with murdering his math teacher as a remorseless killer, while his attorney portrayed him as mentally disturbed.

The suspect, Philip Chism, is being tried as an adult in the murder of Colleen Ritzer, 24, who had been a popular teacher at Chism's high school in Danvers, Massachusetts, a town of 26,000 about 20 miles north of Boston.

"The defendant arrived at Danvers High School with a mask, a box cutter and a terrible purpose," said Kate McDougall, an Essex County prosecutor.

Chism's attorney said the teen, a freshman at the time, attacked Ritzer on Oct. 22, 2013, due to mental illness.

RELATED: Danvers teen accused in teacher killing competent to stand trial

"Why did a 14-year-old boy, well behaved, quiet, one month into high school, do these terrible things?" said defense attorney Denise Regan. "The answer is he was severely mentally ill. He was suffering from psychotic disorder since age 10."

Prosecutors contend that Chism followed Ritzer into the bathroom after school, raped her, cut her throat with a box cutter and transported her body off campus in a recycling receptacle.

Chism is also accused of taking Ritzer’s credit card, which prosecutors say he used to buy fast food and a ticket to a movie at a mall.

A Topsfield, Massachusetts, police officer who found Chism walking along a rural road the night of Oct. 22 testified that he noticed blood on Chism’s clothing after bringing him to the police station.

RELATED: Trial to begin in case of teen who allegedly raped, murdered Danvers teacher

The officer, Joseph DeBernardo, asked about items Chism had in his backpack, including a box cutter with blood on it inside a white wallet.

DeBernardo said he asked whose blood it was. He said Chism replied, “The girl’s.”

“I asked him if he knew where the girl was,” DeBernardo said. “He stated she was buried in the woods. I asked if we could help her. He said no.”

Chism is being charged as an adult for first-degree murder. He has been charged as a juvenile for two counts of aggravated rape and armed robbery.

If found guilty of first-degree murder, he would face a mandatory sentence of life in prison without parole.

Chism, who was known as quiet and a talented soccer player, was described at the time as being under emotional strain after relocating to Danvers from Tennessee with his mother following his parents’ divorce.