SALEM, Mass. — A teenager accused of killing his high school math teacher told police he knocked her out with a “karate chop” before slicing her neck two times with a box cutter, a police detective said Friday in a pre-trial hearing.

But Philip Chism, now 15, denied raping the popular teacher, Colleen Ritzer, before killing her on Oct. 22, 2013, Danvers Det. Sgt. Phil Tansey said.

Tansey said Chism also helped investigators recover Ritzer’s body by showing on a map where he dumped it in the woods behind the school.

The detective’s statements come as a judge is considering a request by Chism’s lawyers to have his confession and other evidence seized from him the night he was detained tossed out as evidence before trial.

Superior Court Judge David Lowy made no ruling; the hearing is expected to continue later this month.

Authorities say surveillance video from Oct. 22, 2013, shows Chism following Ritzer into a school bathroom, wearing gloves and a hood, then later walking out of the bathroom alone.A short time later, the video shows Chism pulling a recycling barrel through the school and outside.

Ritzer’s body was later found in nearby woods, naked from the waist down and with her throat slit and a note that read, “I hate you all.”

A recycling barrel was found near Ritzer’s body. Authorities said she was sexually assaulted with a stick.

Earlier Friday, prosecutors called on state Police Detective Lt. Norman Zuk, who testified that Chism said a “trigger word” had set him off before the murder. Zuk did not say what that word was.

Defense lawyers, meanwhile, called on Dr. Thomas Grisso, a retired University of Massachusetts professor and expert in juvenile psychiatry.

He testified about the ability of juveniles to understand the implications of waiving their right to not speak with police.

Grisso said adolescents tend to make decision based on “what will that get me now” and can be more easily swayed than adults by the prospect of short-term benefits, such as getting to go home, sleep or eat.

Chism’s lawyers say police coerced his mother, Diana, to get her then-14-year-old son into waiving his rights and making detailed statements about the murder.