NASHUA, N.H. — A 21-year-old man who admitted he took part in a machete and knife attack on a New Hampshire woman and her daughter was convicted of murder today, after jurors rejected his claim of insanity.

Minutes after the jury returned the verdict against Christopher Gribble, New Hampshire Judge Gillian Abramson imposed the mandatory sentence of life without parole, telling Gribble, “infinity is not enough jail time.”

For the first time in the proceedings, victim Jaimie Cates, now 12, appeared in the courtroom and witnessed the sentencing. Most of Gribble’s knife blows targeted her.

Abramson thanked her for her presence in the courtroom and assured her that the men involved in this “horrible crime” could never hurt her again.

The judge closed by telling Jaimie that she wished her better days.

The jury deliberated approximately two hours over two days before finding him guilty of two counts of first-degree murder, attempted murder, conspiracy to commit murder, conspiracy to commit burglary and witness tampering.

David Cates, the victim’s husband, left the courtroom after the verdict was read with his lips pursed.

Gribble admitted he took part in the Oct. 4, 2009, home invasion and that he and co-defendant Steven Spader intended to kill anyone they found in the house. Spader was convicted in November and is serving a life sentence.

Gribble took the stand in his own defense during the 11-day trial, claiming he had been abused by his mother and that he had fantasized about torturing and killing her. He asked a jury to find him not guilty by reason of insanity, but prosecution experts who examined him testified Gribble had an anti-social personality but knew the difference between right and wrong.

Juries in New Hampshire have wide latitude to determine what a mental disease or defect is and whether it was the cause of the crimes committed. It has been more than half a century since a New Hampshire jury returned a verdict of not guilty by reason of insanity.