SALEM (CBS) — The defense rested its case in the Philip Chism murder trial on Tuesday as psychologists called by prosecutors testified that the teenager was faking the extent of his mental illness.

Chism, 16, is charged with raping and killing his 24-year-old math teacher, Colleen Ritzer, in 2013.

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Colleen Ritzer. (Facebook photo)

Colleen Ritzer. (Facebook photo)

Prosecutors allege that Chism attacked Ritzer in a Danvers High School bathroom. Her body was discovered in a trash bin in a wooded area behind the school.

Chism’s lawyers are pursuing an insanity defense and say the teenager hears voices in his head.

But witnesses for the prosecution called that strategy into question on Tuesday, saying that their tests showed Chism was faking the extent of his mental illness.

“He does have a fantasy life in that he might choose to retreat to that when he’s involved in stressful situations,” Dr. Kelly Casey said. “But he does seem to understand the difference between reality and fantasy, and he didn’t show any signs of getting lost in a fantasy world.”

Chism has answered hundreds of questions from psychologists since the murder.

Dr. Nancy Hebben, another prosecution witness, said it’s very easy to feign mental illness.

The case is expected to be in the hands of a jury by Thursday or Friday.

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WBZ NewsRadio 1030’s Bernice Corpuz reports: