GRAHAM, Wash. (AP) — Arson investigators are going through what's left of a house in Washington state where the husband and two young sons of a missing Utah woman died yesterday.

Officials say they believe Josh Powell set the fire that killed him and the boys, ages 5 and 7. They're trying to determine exactly how the fire began and what accelerants might have been used.

The fire erupted after a social worker arrived with the boys for a supervised visit.

The social worker told authorities Powell let the boys in but locked her out. She called her supervisors to report smelling gas. Moments later, the house burst into flames.

An assistant chief says responders arrived on scene about three minutes after getting the call and found flames already through the roof.

The Washington Department of Social and Health Services says the social worker is "suffering from grave emotional trauma as a result of the horrific event."

Powell had been described as a "person of interest" in the disappearance of his wife, Susan Powell, in 2009. The boys were living with her parents.