MODESTO, Calif. >> A central California man has died after he and his 77-year-old mother were mauled by a pack of pit bulls owned by his neighbors, sheriff’s officials said Wednesday.

The 59-year-old man died of multiple traumatic injuries suffered in Tuesday’s attack at his home in the city of Modesto, Stanislaus County Sheriff Adam Christianson said. His mother remained hospitalized in critical condition.

Investigators determined that the dogs were owned by the victims’ neighbors and got into the victims’ yard through a common fence, Christianson said. He did not identify the owners. They have not been arrested, though Christianson said the investigation into the attack was continuing.

“There is still a lot of work to do,” he said. “We have to be able to finish the investigation to determine what happened that led up to this tragedy.”

To charge the dog owners criminally, Christianson said authorities would have to show they were negligent. Investigators will be looking to see whether the animals had any contact with animal control officers prior to the attack over bites or vicious behavior.

Deputies responding to several 911 calls found the man being mauled by three pit bulls in the backyard of his home and a fourth pit bull circling him. Deputies shot and killed two of the dogs. The other two ran off to an adjacent yard but were later shot to death when they charged deputies trying to capture them, sheriff’s officials said.

The mother was found inside the home suffering from bite wounds and told deputies she, too, had been attacked by the dogs.

Both victims, who were not identified, were taken to a hospital.