John Bacon

USA TODAY

A Massachusetts teen accused of murdering his teacher in a school bathroom last year has been charged with attempting to kill a juvenile detention staffer the same way.

A Boston grand jury indicted Philip Chism, 15, in the assault last month on a staffer at a Metro Youth Services facility in Dorchester, Suffolk County District Attorney Daniel Conley said. Chism, of Danvers, is accused of attempted murder by strangulation, assault with intent to murder, kidnapping, and two counts of assault and battery with a dangerous weapon.

Conley said Chism followed the 29-year-old staffer into a bathroom and physically assaulted her June 2. Co-workers heard her cries for help and interrupted the assault, Conley said. State Police are investigating the case.

The charges were filed in juvenile court, but Conley said Chism could face adult penalties. Chism remains in custody; arraignment on in the Suffolk County Juvenile Court had not yet been scheduled.

Chism was in custody after pleading not guilty to robbing, raping and killing Danvers High School math teacher Colleen Ritzer, 24, after following her into a school bathroom in October. Her body was found in the woods, naked from the waist down and partially buried in leaves. Her throat had been slit, and a note read, "I hate you all," according to a search warrant application.

Chism was a 14-year-old soccer player who had moved to Massachusetts from Tennessee weeks before the attack. A student whose name was concealed in the documents told police that the day of the killing, she had stayed after class for help and heard Ritzer and Chism talking.

According to the documents, surveillance video showed Chism putting on gloves and wearing a hood as he followed Ritzer into a bathroom. Officials say she had been sexually assaulted with a stick. The documents say Chism brought a box cutter, mask, gloves and multiple changes of clothing to school that day.

A timeline in the documents, based on surveillance video indicates a student may have seen part of the crime. As Chism and Ritzer are apparently in the bathroom together, a female student walks in, then quickly walks out. The student later said a person with dark skin appeared to be changing clothes. Chism is biracial; Ritzer was white.

One minute later, Chism walks out of the bathroom wearing a hood over his head. Nine minutes after that, he pulls a recycling barrel into the bathroom, then through the school and outside.

Chism, who had been spotted at a movie theater after the killing, was found walking along a highway in a neighboring town around 12:30 a.m. A police officer looked in his backpack and found a bloodstained box cutter, according to the documents.

Asked where it came from, Chism replied, "The girl."

Contributing: Associated Press