A student (14) in the US state of Massachusetts accused of killing his teacher has now also been charged with raping her.

Philip Chism has been held since shortly after the October 22nd killing of Colleen Ritzer, a maths teacher popular among students and colleagues at the Danvers High School.

Chism allegedly murdered the 24-year-old in the school and then dumped her body in the woods nearby, where it was later found. He has already been indicted on charges of murder. Now authorities have added charges of aggravated sexual assault and armed robbery.

Authorities say he sexually assaulted Ms. Ritzer with an object before killing and robbing her.

"The indictments returned today detail horrific and unspeakable acts" Essex County District Attorney Jonathan Blodgett said. More details could be made public later.

The 24-year-old was allegedly murdered in the school where she worked

According to the indictments, Chism, armed with a box cutter, stole Ms. Ritzer's underwear, credit cards and iPhone. Chism has pleaded not guilty to the murder charge.

Ms. Ritzer's family said in a statement that they continue to mourn and asked for privacy.

"We are devastated and heartbroken by the details of the horrific circumstances surrounding the death of our beautiful daughter and sister, Colleen" the statement said.

Classmates said Ms. Ritzer had asked Chism to stay after school on the day that she was killed but a motive remains unclear.

Chism had moved to Massachusetts from Tennessee before the start of the school year and played on the school's junior varsity soccer team.