Josh Powell Told Son He Had 'Big Surprise' Moments Before Killing Children (VIDEO)

02/09/2012 11:09 pm ET | Updated Apr 10, 2012

SEATTLE — The Washington state social worker who was supposed to supervise a visit between Josh Powell and his young sons says Powell told his oldest boy he had a "surprise" for him moments before attacking and killing the children.

Elizabeth Griffin-Hall said in an interview to air Friday on ABC's "20/20" that Powell slammed the door on her Sunday after he had the children inside the house. Hall said she banged on the door to try to get inside and heard Powell tell 7-year-old Charlie, "I've got a big surprise for you." She also heard 5-year-old Braden crying.

Authorities said Powell used a hatchet on his children, then set a house fire that killed them all.

Powell's wife, Susan, vanished in Utah two years ago. Josh Powell had long been a person of interest in the case but maintained that he had taken his boys – then 2 and 4 – on a midnight camping trip in freezing temperatures when she disappeared from their home.

Late Thursday, authorities told The Associated Press a computer in Josh Powell's Utah home two years ago had animated images that depicted "incestuous" sex. Pierce County Sheriff's Detective Ed Troyer said the images collected by investigators were realistic computer-generated depictions of incestuous parent-child relations. The images were disconcerting enough that they prompted a psychologist to recommend last week that Powell undergo an intensive psychosexual evaluation.

An attorney for Powell's in-laws wasn't invited to see the materials before a hearing last week. Lawyer Steve Downing said he might have asked the court to change the terms of Powell's supervised visitation with the boys if he had seen the images.

Josh Powell had lost custody of the boys last fall, after his father, with whom they lived at the time, was arrested in a child pornography and voyeurism investigation.

On Sunday, the social worker drove the boys from their maternal grandparents' home to Josh Powell's house outside Puyallup, about 35 miles from Seattle.

Griffin-Hall said Charlie and Braden loved being with their father.

"One of them said what he wanted to do was go home and live with his daddy," she told ABC, adding that the boys would "light up" during visits with Josh Powell.

After he got the boys inside and locked the door, Griffin-Hall called her supervisor and then 911 to tell them what was going on. The 911 operator's handling of that call has been criticized, and an investigation has been launched into the emergency response.

Logs show deputies weren't dispatched until eight minutes after Griffin-Hall's initial contact with authorities, though police say any delay would not have stopped what ultimately happened to the boys.

The social worker told ABC that she told her boss "something terrible is happening here, and I was on the phone with ... when the house exploded.

"I wanted to get to the kids," she said. "I wanted to get to the kids. I would have broken in if I could."

But Griffin-Hall said she doesn't think she could have saved them.

"How this happened is that Josh Powell was really, really evil. I couldn't have stopped him," she said. "I did everything I was supposed to do. I did everything right and the boys are still dead."

She said she loved the boys and was like a "grandma" to them.

"The world lost two beautiful boys to a monster," Griffin-Hall said.