Police affidavit offers chilling details of teacher's slaying

(CNN) -- Philip Chism, the Massachusetts teenager accused of raping and killing his algebra teacher, became visibly upset when the teacher, Colleen Ritzer, spoke about the teen's home state of Tennessee after class, according to a police affidavit unsealed Friday.

A ninth grade student told investigators that she was in class with Chism and Ritzer after school on the day of the crime, the affidavit said. She said the teacher and Chism were talking about China but, at some point, Ritzer mentioned the student's home state of Tennessee.

Chism became "visibly upset," the student said. When Ritzer noticed that Chism was upset, she changed the subject, said the unidentified student, who described Chism as "talking to himself."

The affidavit, in chilling detail, offers the first hint of a possible motive in last month's gruesome killing of the popular high school teacher. Ritzer, 24, was allegedly raped with an object and had her throat slashed. A handwritten note found next to her body said, "I hate you all."

A Massachusetts grand jury on Thursday indicted Chism, 14, on charges of murder, aggravated rape and armed robbery in connection with Ritzer's slaying on October 22, according to the Essex district attorney. Citing Massachusetts law, prosecutors said they would ask that Chism be tried in an adult court.