A 21-year-old man admitted in court Monday that he hacked a mother to death and seriously wounded her young daughter during a 2009 home invasion but said he was insane when he did it.

Christopher Gribble of Brookline waived his right to a jury trial on whether he killed 42-year-old Kimberly Cates and seriously wounded her 11-year-old daughter, Jaimie.

The burden now shifts to Gribble's lawyers to prove he was insane during the Mont Vernon home invasion.

Gribble told Superior Court Judge Gillian Abramson he was diagnosed in 2007 as having "anti-social and personality disorder" and that a doctor prescribed the antidepressant Prozac. He said he discontinued the drug on his own. That was the only reference during the 90-minute hearing to his mental state.

If the jury finds he was sane at the time, Gribble will be sentenced to life in prison without the possibility of parole.

If jurors find him not guilty by reason of insanity, Abramson would hold a hearing on whether he is a danger to society. If she finds that he is a danger, she will commit him to the secure psychiatric unit of the New Hampshire State Prison. He would then be entitled to have a review of his threat to society every five years.

Gribble answered, "Yes, your honor" when asked if he committed murder, attempted murder, conspiracy to commit murder and burglary, and tampered with a witness. He then pleaded not guilty by reason of insanity to each charge.

Gribble listened intently as Senior Assistant Attorney General Jeffery Strelzin detailed the evidence the state was prepared to present. It mirrored the evidence that was presented to the jury that convicted 19-year-old Steven Spader of first-degree murder last month.

Prosecutors say Spader wielded a machete and Gribble a knife in the attacks on the mother and daughter. A medical examiner testified during Spader's trial that Kimberly Cates was alive while all 32 blows from the knife and machete ripped through her body. Her daughter suffered 18 wounds and lost a portion of a foot in the attack.

Gribble opted for a jury trial on the insanity claim. Jury selection is scheduled to begin in February.

Also Monday, Gribble's lawyers argued to suppress a detailed confession he gave to investigators the day after the attacks. They claim investigators improperly told Gribble they were offering him "a lifeline" if he talked with him.

State police Sgt. John Encarnacao testified that Gribble was emotionless and showed no sign of empathy as he detailed the gruesome attacks.

When Assistant Attorney General Peter Hinckley was arguing against suppressing the confession, Gribble appeared to grow angry and animated as he spoke with defense attorney Matthew Hill.

Abramson said she would rule soon on the confession.