Josh Powell, the Washington-state man who killed himself and his sons by blowing up his home Sunday, told his sons that he had a “big surprise” for them moments before the explosion, said social worker Elizabeth Griffin Hall in an ABC News interview. The social worker was taking the boys, ages 5 and 7, to visit their father, who lost custody of his children in September. He lost an appeal to regain custody four days before killing his sons. Powell locked Hall outside his home Sunday after taking his children in. Hall heard Powell promise a “surprise” as she banged on the front door, then heard the youngest boy cry out. Hall smelled gas, and was on the phone with her boss when the house exploded.