Danvers, Massachusetts (CNN) -- Philip Chism, the 14-year-old student charged in the slaying of Danvers, Massachusetts, high school teacher Colleen Ritzer, allegedly used a box cutter to kill her, two law enforcement sources close to the investigation told CNN's Pamela Brown Thursday. A source close to the investigation said Chism brought the box cutter to school with him. After killing Ritzer in a school bathroom, the source said, Chism allegedly rolled her body out of the building in a recycling bin and dumped her in the woods nearby. The two law enforcement sources say Chism changed his clothes after killing the teacher. The ordeal started Tuesday as a missing persons case. Authorities first searched for Chism after he was reported missing. They alerted the public Tuesday evening that the teen had been last seen around 6:30 p.m. But around 11:20 p.m., they got a call that 24-year-old Ritzer had also disappeared. She hadn't returned home and was not answering her phone, said Essex County District Attorney Jonathan Blodgett. Police in Topsfield, the town just north of Danvers, spotted Chism walking alongside a busy road there around 12:30 a.m. At Danvers High, police found blood in a second-floor bathroom, the district attorney said. The teacher's body was soon discovered behind the school. Detectives interviewed Chism, and whatever the teen told them about what went on at the school and in the surrounding woods, coupled with video surveillance, led to an arrest.