Salem, Mass. (AP) — A 15-year-old Massachusetts boy charged with killing his math teacher is returning to court Thursday for arraignment on a second rape charge.

Philip Chism is charged in the October killing of Colleen Ritzer, a 24-year-old teacher at Danvers High School.

Chism has pleaded not guilty to charges of rape, robbery and murder. He was indicted last week on the initial rape charge. In court documents filed last week, state police say Chism admitted killing Ritzer but denied raping her.

Police say they want to conduct a forensic examination of Chism's cellphone to see if he memorialized the killing in photos, video or audio recordings.

Authorities allege Chism raped and killed Ritzer after she asked him to stay after school for extra help.

Chism's arraignment is in Salem Superior Court.