Jurors in the trial of a deadly machete-and-knife attack on a New Hampshire mother and daughter began to listen Tuesday to a seven-hour police interview of the man who admits he committed the crimes.

Christopher Gribble, 21, at first denied hacking to death Kimberly Cates and attempting to kill her 11-year-old daughter, Jaimie. But he changed his story and confessed in chilling detail once he learned a co-conspirator was cooperating with police.

Gribble is trying to convince the jury he is not guilty by reason of insanity.

Gribble and co-conspirator Steven Spader were known to investigators the day after the Mont Vernon home invasion. Both had bragged extensively about the attacks to a group of friends, one of whom told the police.

State Police Sgt. John Encarnacao testified Tuesday that he caught up with Spader and Gribble outside Spader's house Oct. 5, 2009, and both agreed to come to the barracks to be interviewed.

Gribble chatted with police for hours about crime shows he watched, but denied any involvement in the home invasion. Gribble admitted his involvement only after it became apparent investigators had inside information about the crimes.

On Wednesday, jurors are expected to hear the confession segment of Gribble's audiotaped interview.

During the first half of the interview, as Gribble denied any knowledge about the crimes, two investigators asked Gribble what should happen to whoever attacked the victims. Gribble said they should probably go to prison for the rest of their lives _ the fate Gribble faces if the jury rejects his insanity defense.

"Assuming someone, somebody, specifically tried to kill them, like that rubs against my conscience personally," said Gribble, who described himself to the investigators as chivalrous. "It really goes against me to harm a woman in general, but a little girl. How could you do that to someone so young?"

Gribble initially told police he was riding around with Steven Spader _ a co-conspirator who was convicted in November of first-degree murder and other felonies and is serving two life sentences. Gribble said the pair watched television at a friend's house, and slept in Gribble's car after the friend's mother became angry he had guests.

The investigators began the interview bantering with Gribble about cars and girlfriends. Their tone gets sharper and questions more focused after Gribble had droned on about random subjects for several hours. But even after Encarnacao tells him "the gig is up ... the story is coming unraveled," Gribble calmly disagrees.

"Then take me away," Gribble tells them. "I didn't do anything."

Gribble has the burden of proving he was insane at the time of the crimes. Prosecutors had the option of not even presenting a case. They are using the taped interrogation and other evidence to show how calculating and intentional Gribble's actions were.

A forensic psychiatrist testified Monday that Gribble is manipulative and proud of his ability to lie.

Dr. Albert Drukteinis testified Gribble has some personality disorders and anti-social traits, but is not legally insane.

A doctor from Boston Children's Hospital on Tuesday testified about the severity of the injuries sustained by the 11-year-old girl.

Dr. Amir Taghinia said the child was in surgery for 12 hours after being air-lifted to the Boston hospital. He said Jaimie could have died from a punctured lung if she hadn't crawled to the kitchen and phoned police.

The doctor said he had never seen a patient with as many puncture wounds as the child sustained.