Preliminary autopsy results show that a pregnant San Francisco-area woman who was killed by her pit bull died from blood loss and shock, police said on Monday.

An autopsy on 32-year-old Darla Napora, along with analysis by dog bite experts, also concluded that only one of her two dogs, a 2-year-old male named Gunner, took part in last Thursday's fatal attack.

Police say Napora's husband returned to the couple's home in the Bay Area village of Pacifica about noon on Thursday to find his pregnant wife's mauled body, with the bloodied pit bull standing over her.

First responders failed to revive Napora, who was suffering from massive trauma to her upper body, police say, and she was pronounced dead at the scene, along with her unborn baby, Pacifica police said.

Gunner was shot and killed by officers in the front yard of the couple's home after he apparently got free from a back room, where Napora's husband had tried to confine him, police said.

A second pit bull, a female, was found cowering in a corner.

Impressions taken from both dogs show that all of Napora's injuries were consistent with bites from Gunner and that there was no evidence that the female dog took part in the attack, Pacifica police said.

Pacifica Police Capt. Dave Bertini said that authorities were still trying to determine why Napora's dog attacked her.

"We may never know why it happened," Bertini said, adding that the couple was keeping both pit bulls as pets.

Bertini said there was so far no evidence to indicate that the dogs had been trained to be aggressive.