Philip Chism revealed further details regarding the killing of his math teacher during a hearing on Friday in Salem Superior Court.
The Massachusetts student was 14 years old when he allegedly raped and killed Colleen Ritzerin October 2013.
The former Danvers High School student completed the second day of testimony to determine whether statements made to police after his mother requested an attorney are admissible.
Chism initially told state police detective Lt. Norman Zuk that there was "some trigger word that got him angry," the day he killed Ms. Ritzer, although he would not reveal what the word was.
He later told Zuk and Danvers police detective Sgt. Philip Tansey that "he had given her what he described as some sort of karate chop" that rendered Ritzer unconscious.
Chism then put a hand over her mouth and "dragged" a box cutter across her throat twice.
The Salem News reported that as Sgt. Tansey testified, Ritzer's mother was nearly doubled over and weeping.
Although Chism denies sexually assaulting his teacher, prosecutors said the evidence indicates he raped her twice in an attack that began in the high school's bathroom.
The teen drew a map for investigators that led them to Ritzer's body, and another map that led them to her discarded cell phone. Near the body, they also found a bloody pair of jeans hanging on branches, and a pair of shoes. It was unclear whose clothing was found.
A box cutter and a diving knife were found in Chism's backpack.
During the hearing, Judge David Lowy heard a recording of Chism's mother, Diana, expressing concern for her son to retain an attorney. An investigator told her that her son didn't want anyone's help, and that he only wanted to "escape" to a juvenile detention centre.
The hearing is expected to continue until the end of the month.
