Just moments before Josh Powell ignited two five gallon gas cans, blowing up his house, himself, and his two boys, be told his sons he had a "big surprise" for them.

In an interview with ABC's 20/20, social worker Elizabeth Hall said he shrugged his shoulders and slammed the door in her face.

She said she banged on the door and heard Powell say 'Charlie, I've got a big surprise for you.' And then I heard Braden cry out."

Autopsies later revealed both boys had chop injuries to their necks and heads, but that they died of carbon monoxide poisoning. Two gas cans and a hatchet were found near the bodies.

Meanwhile, Hall said she realized she didn't have her phone. She went to move her car, because she could smell gas, and call 911.

After hanging up with 911, Hall said she was on the phone with her boss when the home exploded in front of her.

"I wanted to get to the kids," she told 20/20. "I wanted to get to the kids. I would have broken in if I could."

Signs of violence have been present before, according to divorce papers from Steven and Terrica Powell, Josh Powell's parents.

ABC News reports Terrica wrote that her son had threatened her with a butcher knife, pushed and hit her (with the help of Josh's brother) and once attempted to commit suicide.

Terrica described Powell as "menacing" and said that she was "terrified" when he threatened her with the butcher knife in response to being asked to do the dishes.

Despite damning evidence against Josh Powell, his sister, Alina Powell, continues to defend her brother to ABC. Alina believes her brother has nothing to do with the disappearance of his wife, Susan Cox Powell.

Alina says she doesn't know why her brother did it. "I can't put myself in his mind," she said. "But I think, he must have been, just more heartbroken than I realized. Then any of us realized."

Before Josh Powell was going to try to win back custody of his children last week, Washington state authorities received a set of materials from Utah police that had been discovered on a computer in Powell's home. The images prompted a psychologist to recommend that Powell undergo an intensive psychosexual evaluation.

Hall, who said she was like a grandma to the Powell boys, said she blames their father for their deaths and knows there's nothing she could have done to prevent what he did.

While she's devastated, Hall said she won't let Powell's evil prevent her from doing something she loves: Working with children.

Hall's full interview airs on 20/20 Friday at 10:00 p.m.

The Associated Press contributed to this report.

MyNorthwest.com, Staff report

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