GRAHAM, Wash. - A powerful house explosion killed the husband of a Utah woman who has been missing for two years, and his two young sons Sunday, moments after the boys arrived for a visit that was supposed to be supervised by a social worker.

The Child Protective Services worker brought the two boys to Josh Powell's home, and Powell let his sons inside - but then blocked the social worker from entering, Graham Fire and Rescue Chief Gary Franz told The Associated Press.

The social worker called her supervisors to report that she could smell gas, and moments later the home exploded. What's left of the house was still smoking Sunday afternoon, and fire crews were mopping up the scene.

"Everything we know right now, this has become a crime scene," Franz said.

It was a horrific end to a long, bizarre tale. Josh Powell was under investigation in the disappearance of his 28-year-old wife from their West Valley City, Utah, home in December 2009. He claimed he had taken the boys on a midnight excursion in freezing temperatures when she vanished.

The children, 4 and 6, had been living with Susan Powell's parents since Josh Powell's father Steven was arrested on child porn and voyeurism charges last fall. On Wednesday, a judge had denied an attempt by Josh Powell to regain custody, saying she wouldn't consider returning the two boys to their father until he underwent a psycho-sexual evaluation.

The case took a bizarre turn last year after Powell's father, Steve, was arrested for investigation of voyeurism and possessing child pornography. Josh was living at his father's home at the time, and a judge gave Susan Powell's parents custody of the boys.

Jeffrey Bassett, who represented Josh Powell in the custody case, said he received a three-word email from his client just minutes before Powell and his two boys died in a house explosion. It said, "I'm sorry, goodbye."

The email arrived at 12:05 p.m. Sunday, about 10 minutes before the explosion, but he didn't see it two hours later, when others told him Josh and the boys had been killed. He said he knew Josh was upset after being ordered to undergo a psycho-sexual evaluation recently, but he didn't see this coming.

Sgt. Mike Powell of the West Valley City Police Department in Utah, which is handling the investigation into Susan Powell's disappearance, said it was too soon to say how Josh Powell's death may impact their probe.

"Quite frankly, this has obviously quickly unfolded up in Washington and we're obviously just working through the details ourselves here," Powell said Sunday.

"We are in contact with authorities," Powell added. "It's obviously an ongoing situation in Washington at this point."