Graham Fire and Rescue Chief Gary Franz said the blast occurred moments after the two boys were dropped off at Powell's home by a Child Protective Services worker. The worker said Powell blocked her from entering the home after allowing the two boys in.
The social worker called supervisors to report that she smelled gas. Shortly after, the home exploded.
Powell's lawyer Jeffrey Bassett said he received a three-word e-mail from his client just minutes prior that read, "I'm sorry, goodbye." Bassett said he didn't see the message until two hours later, when others informed him of the blast. He said he knew Josh was upset after being ordered to undergo a psycho-sexual evaluation, but did not see this coming.
Powell is the husband of Susan Powell, a Utah woman who has been missing since 2009.
The Associated Press contributed to this report.