UNION CITY, Calif. — A 6-year-old San Francisco Bay Area boy died after he was attacked by a dog that belongs to his uncle, who is a police officer, authorities and a family spokesman said Tuesday.

The dog, described as a pit bull or pit bull mix, bit Nephi Selu of Dixon on top of his head Monday morning at his grandparents' home in Union City, police said.

Relative pulled the dog off the boy, said Union City police Cmdr. Ben Horner.

Emergency crews took Nephi to Lucile Packard Children's Hospital in Palo Alto, where he was declared dead about 4 1/2 hours after he was mauled.

Family members told investigators the boy was playing with the dog and may have attempted to climb on its back when he was attacked, Horner said.

They said the dog had never shown any signs of aggression.

"He played with kids, very obedient, he was never allowed in the house, stayed right at the door, so we never had any problems with him at all," the boy's aunt, Iona Keanaaina, told KGO-TV.

Attorney Michael Rains, who works for the San Mateo police union and is acting as a family spokesman, said the dog's owner, whose name he did not disclose, believed the boy's injuries were not very serious and went to work before learning he had died. He said the owner lived at the house, as did the victim and his mother.

The dog was taken to an animal shelter for a routine, 10-day quarantine, Horner said. The dog was in compliance with anti-rabies shots and licensing requirements, Horner added.

Rains told the Oakland Tribune the attack was not a case of criminal liability, just a terrible accident that happened to a nice family.

"This dog has never even barked at these kids," Rains said. "You can't figure out what happened. He was just playing with him and the next think that happened is the dog bit the child."