Prosecutors have asked for strict security for a Danvers teenager charged with killing his teacher after he allegedly attacked a female youth worker this month.

In court papers released Thursday at the request of The Salem News, Essex County prosecutors said 15-year-old Philip Chism slipped away from caregivers before the June 2 incident at a youth facility in Boston.

Prosecutor Kate MacDougall said Chism crept along a common hallway "crouched down out of view," followed the woman to a locker room and tried to choke her while holding a pencil. Other workers came to the woman's aid when she screamed.

Chism is at Worcester State Hospital for evaluation.

Chism pleaded not guilty to robbing, raping and killing 24-year-old Danvers High School teacher Colleen Ritzer after following her into a restroom last October.

Ritzer had not returned home from work and was not answering her cellphone. When investigators went to the school to look for her, they found blood in a second-floor school bathroom. Chism, who had also been reported missing, was found by police walking along Route 1 in nearby town of Topsfield.

Joe Spanos, a former teacher at Andover High School, knew Ritzer for about 10 years. He mentored her, she worked at his convenient store and, at times, babysat his children. Spanos said Ritzer was the kind of woman that you wanted your daughter to emulate.

"She was beautiful on the inside and out," he told FoxNews.com. "She's not a fighter. She probably never saw it coming," he said.

Ritzer appeared to have a Twitter account where she posted math questions and assignment for students. On Aug. 11, she posted, "No matter what happens in life, be good to people. Being good to people is a wonderful legacy to leave behind."

The Associated Press contributed to this report