A judge heard arguments and testimony Friday as defense lawyers seek to have a teenager's police confession that he killed his math teacher kept out of his trial.

State police Detective Lt. Norman Zuk, who was among the officers that interviewed Philip Chism and his mother the night he was arrested, testified that Chism said a "trigger word" had set him off before the murder. Zuk did not say what that word was.

Chism, 15, is charged as an adult with murder and aggravated rape in the October 2013 slaying of Colleen Ritzer, a teacher at Danvers High School. He has pleaded not guilty.

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

Prosecutors maintain Chism, who had recently moved to Massachusetts from Clarksville, Tennessee, and his mother never absolutely invoked their right to a lawyer. They said Diana Chism considered it but ultimately left her son to decide if he wanted to speak with law enforcement.