Authorities told the Los Angeles Times the conflagration occurred shortly after a social worker dropped off Josh Powell's sons, Charlie, 7, and Braden, 5, at the home for a scheduled supervised visit.
Graham Fire and Rescue officials said the social worker called her supervisors to report she could smell gas when the explosion happened, the newspaper said.
"She moved away from the house, a very smart move on her part, and moments to minutes later, the house exploded," Graham Fire and Rescue spokesman Gary Franz told the Times.
Franz said "credible reports" from witnesses suggested the Powell children were inside the house. Fire officials found the bodies of an adult and two children, assumed to be Powell and his sons, Franz said.
"Everything points to Josh Powell and his two sons," he said.
Authorities had long considered Powell a person of interest in the disappearance of his wife, who hasn't been seen since she vanished from the family's home in West Valley City, Utah, in December 2009 while he and the two boys were on a camping outing during a snow storm.
Powell moved to Washington state with the boys but Susan Powell's parents won custody of the children after thousands of sexually explicit pictures and videos of women and young girls were discovered on the computer of Josh Powell's father, Steven Powell.
A judge last week ruled Josh Powell would have to undergo a psychosexual evaluation as part of his bid to regain custody.