(CNN) -- Philip Chism, the Massachusetts high school student who is accused of raping and killing his math teacher, allegedly attacked an employee at a youth services facility in Dorchester, where he is being held pending his trial, the Essex County Sheriff's Department said Tuesday.
The female employee is now at Boston Medical Center with a security detail, according to Maurice Pratt, the assistant superintendent and spokesman for the sheriff's department.
"It's an ongoing investigation," Pratt said of Monday's attack.
"The Department of Youth Services is working closely with law enforcement to investigate an assault that took place on a member of the staff," said a statement from DYS communications director Alec Loftus.
"The safety and well-being of our staff is of primary concern and a full review of this incident will be conducted to identify any necessary security enhancements at the facility," the statement said.
The extent of the employee's injuries or what prompted the alleged attack were not provided.
Chism, 15, is accused of killing his algebra teacher, Colleen Ritzer. Chism, 14 at the time, used a box cutter he'd taken to school to kill Ritzer, 24, on October 22 in a girls' bathroom at Danvers High School, police and prosecutors said.
Chism pleaded not guilty to aggravated rape, robbery and murder charges in January. He is being tried as an adult.
Calls to Chism's attorneys -- Susan Oker and Denise Regan -- were not immediately returned.
After Chism was reported missing by his mother on the evening of October 22, police had his cellular phone company "ping" the location of his phone. The phone was found to be near the Hollywood Hits Theater, where investigators learned the teen had purchased a movie ticket and then left.
A court affidavit said that when Chism was spotted by a police officer the next day, he was carrying a knife; a search of his backpack turned up a bloodstained box cutter. Asked where the blood came from, Chism allegedly responded: "The girl." He was also allegedly carrying credit cards and driver's licenses belonging to Ritzer, as well as a pair of women's underwear.
Police discovered Ritzer's body in a wooded area near the school, covered with leaves and debris in an apparent attempt to conceal it, court papers said. Her throat was slit.