(CNN) -- A 15-year-old Massachusetts boy accused of raping and killing his high school math teacher admitted to the crime, according to a search warrant affidavit released Monday.
Philip Chism was indicted Friday on an additional count of aggravated rape in connection with the death of Colleen Ritzer, 24, on October 22 in the girls' bathroom of Danvers High School. He will be arraigned on the new count on Thursday.
Chism pleaded not guilty in December to murder, aggravated rape and robbery charges stemming from the killing of the popular teacher. Police said a box cutter Chism had taken to school was the murder weapon.
The affidavit, released Monday, said that after being booked in connection with the crime, Chism "admitted to murdering" his teacher. Chism, in the presence of his mother, Diana Chism, waived his Miranda rights.
The affidavit also said Chism may have sought to "memorialize" the crime by photographing or videotaping parts of it with his cell phone, which was later destroyed in what investigators said was an attempt to hide incriminating evidence.
The Essex County district attorney's office said Friday that it presented a grand jury with additional evidence. The new youthful offender indictment alleges that Chism forcibly had sexual intercourse with the victim.
"Aggravated rape is charged when there is serious bodily harm and/or when the rape was committed during the commission of another felonious and assaultive crime," the district attorney's office said in a statement.
Denise Regan, Chism's public defender, could not be reached for comment.
Chism, who is being held without bail, is set to appear January 30 in Salem Superior Court for a pre-trial conference.
According to a police affidavit, a ninth-grade student told investigators that Chism became visibly upset when Ritzer spoke after class on the day of the crime about his home state of Tennessee.
When Ritzer noticed that Chism was upset, she changed the subject, said the unidentified student, who described Chism as "talking to himself."
The affidavit includes testimony from witnesses as well as a school video surveillance timeline showing Chism and Ritzer in the same area of the school during the teacher's final moments.
In the video, Ritzer appears to enter a second-floor girls' restroom -- apparently, a faculty restroom was occupied -- and Chism, wearing gloves and red sweatshirt with the hood pulled over his head, appears to enter the restroom about a minute later, according to the affidavit.
Shortly after, a female student enters the bathroom and then quickly walks out, according to the court papers. She told investigators she saw the back of a person who appeared to be changing clothes; the person's rear was exposed, with clothes piled on the floor.
The video shows Chism leaving the restroom, returning later with a recycling barrel, and again leaving the restroom pulling the barrel -- this time with a black mask on his forehead, the court papers say. He pulls the barrel outside the building and toward the student parking lot. Investigators said the video shows what appear to be blood stains near the bathroom and on Chism's pants.
After Chism's mother reported him missing on the evening of October 22, police had his cell company "ping" the location of his phone. The phone was found to be in the vicinity of the Hollywood Hits Theater, where investigators learned the teen had purchased a movie ticket and then left.
The 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 woman's underwear.
The armed robbery indictment alleges that Chism robbed Ritzer of credit cards, an iPhone and her 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, the court papers said. Her throat was slit.