SEATTLE - The social worker who was supposed to supervise a visit between Josh Powell and his 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 on ABC's "20/20" on Friday that Powell slammed the door on her after he had the children inside the house on Sunday. 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.

On Sunday, the social worker drove the boys from their maternal grandparents' home to their father's house outside Puyallup, about 35 miles (56 kilometres) from Seattle. Josh Powell lost custody of the boys last fall, after his father, with whom they then lived, was arrested in a child pornography and voyeurism investigation.

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 police and her supervisor to tell them what was going on.

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 said 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 told ABC 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.