VISTA -- It took a jury less than an hour on Monday to find that a Fallbrook man was sane when he fatally stabbed his mother-in-law and teenage sister-in-law in a grisly attack nearly three years ago.

The finding means that Jason Duane Cooper -- who the prosecutor called "angry" and "hateful" -- probably will spend the rest of his life in prison without the possibility of parole. His sentencing is set for July 9.

Two weeks ago, the same jury found Cooper, 27, guilty of first-degree murder with the added allegation of torture in the April 26, 2006, slayings of his wife's mother and sister in their gated, ranch-style Fallbrook home.

Trials like Cooper's, in which the defendant pleads not guilty by reason of insanity, consist of two phases under California law. First, the jury must determine guilt; then it must consider whether the defendant was insane at the time.

On Monday, Cooper, wearing a blue dress shirt, watched without expression as his attorney, Wil Rumble, and prosecutor Kim Lagotta laid out their respective arguments regarding his sanity.

The burden was on the defense to prove, by just a slight tip of the scale, that Cooper was insane when he killed the women. They must first prove that the defendant suffered from a mental defect. They then must show that the defendant did not know or understand the nature of his act or that it was morally and legally wrong.

Jenna Liebner, 16, was stabbed 56 times. Her mother, Robyn, suffered 27 stab wounds. Each suffered substantial slice wounds to their neck and face area.

Cooper admitted to killing them. He was the one who placed a 911 call from the home immediately after the slayings.

His father-in-law came home to find Cooper on the floor, rocking back and forth, and his wife and daughter dead in the foyer.

The attack on Jenna Liebner probably came first. Crime scene photographs show her bloody handprints on the carpet as she tried to crawl away, and Cooper's bloody shoeprints following the same path as he repeatedly plunged the knife into her.

Also, Cooper was wearing latex gloves during the attack. But defense attorney Rumble told the jury that the latex gloves just happened to have been in Cooper's pockets when he visited the house to discuss medieval literature with Jenna. He said an argument between his client and Jenna over books started the attack.

The defense contended that Cooper suffered from severe post-traumatic stress disorder from a horrific childhood, one that reportedly included starvation, severe neglect and seeing his father stabbed in the chest.

Cooper has a history of self-mutilation, often cutting himself with a knife in times of great stress, according to testimony.

Prosecutor Lagotta told the jury that Cooper has personality disorders, not any full-blown mental illness.

"He's an angry, hateful, entitled man," Lagotta told the jury. "He went into that house with those attributes and did what he intended to do.

"The defendant knew this was morally wrong and legally wrong," she said.

Contact staff writer Teri Figueroa at (760) 740-5442 or tfigueroa@nctimes.com.