A birthday party in the Las Vegas area ended in tragedy Saturday when a 1-year-old boy died hours after he was attacked by his family's pet dog.

The boy, who was not named, was celebrating his birthday late Friday at his grandmother's home in suburban Henderson, 17 miles southeast of Las Vegas, when he crawled over to the dog and starting petting him.

The 120-pound Mastiff/Rhodesian mix latched its teeth around the boy's head and began shaking him, the Las Vegas Sun reported, citing police.

Family members rushed to the boy and attempted to free him from the 6-year-old dog.

The 1-year-old was transported to a local hospital before being flown by helicopter to University Medical Center's trauma unit in downtown Las Vegas.

Police said the boy, who turned a year old Thursday, died at 1:45 a.m. Saturday local time.

The family said they had owned the dog since it was a puppy and it had never before displayed any signs of aggression. They voluntarily handed over the animal to Henderson Animal Control officers, who are expected to put it down in the coming days.