UNION CITY -- The mother of a 6-year-old Union City boy killed by a family dog said Thursday that the attack felt like a betrayal.

"It was like one best friend turning on the other," Tilema Selu told KGO-TV. "He had no signs of being hostile. In fact, we felt confident that it was a protector of our children."

Selu said her son, Nephi Selu, adored the pit bull mix that killed him at his grandparents' home in Union City on Monday. The dog was euthanized Tuesday.

She said Nephi, her only child, was a "very loving" and "very welcoming" boy who had just finished kindergarten last week. His funeral is planned for next week.

Selu confirmed earlier suggestions that the boy had been riding the dog like a horse just before he was bitten on top of the head. He was declared dead at a hospital in Palo Alto about 4 1/2 hours later. Family members say Nephi was conscious and talking after the attack, and they thought he would need only minor medical attention.

The dog's owner and Nephi's uncle, Keala Keanaaina, said he bought the pit bull mix two years ago thinking, as the boy's mother did, that it would help keep his family safe.

"We brought the dog into our home as a pet to help protect life," said Keanaaina, a San Mateo police officer. "By no means did we know that it was going to take life."