Tacoma, Washington

ABC's Tahman Bradley Reports

We are learning new details behind the fiery tragedy in Washington where Josh Powell took the lives of his little sons.

A Memorial service for the two boys will be held Saturday in Tacoma.

In an ABC News exclusive, the social worker who brought Charlie and Braden Powell to see their father last Sunday says the moment the door opened she
knew something was wrong.

In the moments before his house blew up, Elizabeth Griffin-Hall says she overheard Josh Powell telling his 7 and 5-year old boys he had a "big
surprise" for them.

Griffin-Hall said, "He just kind of shrugged his shoulders up at me like that and then he slammed the door.  And I thought that was an accident, that he didn't mean to lock me out.  And so I knocked, and I knocked and I rang the doorbell and I started yelling at him."

Then, she says, she smelled gas and called 911.

Authorities say Powell attacked his two young boys with a hatchet and then set fire to his house.

Powell's sister, Alina, defended her brother in an ABC News exclusive.

Alina Powell said, "I think he may have just felt that there was only one way left for him to protect his sons from all the emotional and physical pain that they've been experiencing."

Josh Powell's wife, Susan, disappeared in December 2009.

Powell was called a "person of interest" by police, but he was never charged with anything.

There were no indications Powell would harm his children, Griffin-Hall says.

The Associated Press is reporting that a computer taken from Powell's Utah home two years ago had incestuous images on it.

The AP reports a psychologist recommended last week that Powell undergo evaluation.