(TACOMA, Wash.) -- Josh Powell told his sons he had a "big surprise" for them as they ran toward his home Sunday, just moments before he attacked them with a hatchet and then blew them up, social worker Elizabeth Griffin Hall told ABC News on Thursday.
"He caught my eye, his shoulders were slumped. He had a sheepish look," Hall told ABC’s 20/20 in an exclusive interview. "He just shrugged his shoulders and slammed the door."
Hall said she had been taking the boys, Charles, 7, and Braden, 5, on supervised visits to Powell's house for three months.
Powell, who was the sole suspect in his wife Susan Cox's disappearance in 2009, had lost custody of the children in September and lost an appeal to get his children back just four days before he decided to kill them.
Hall said the children loved being with their father. "One of them said what he wanted to do was go home and live with his daddy," she said.
And during visits with Powell, "I would see them light up."
Watch "Sins of the Father" on ABC's 20/20 Friday at 9 p.m. ET for more. A preview of the report, including more with social worker Elizabeth Griffin Hall, will air on Thursday night's World News with Diane Sawyer and Friday's Good Morning America at 7 a.m. ET.
On Sunday, the children bounded out of her car and took off running for their father's house with Hall a few steps behind. Powell let them in, gave Hall his odd look, and locked the door.
As Hall banged on the door, "I heard him say, 'Charlie, I've got a big surprise for you'... And then I heard Braden cry out."
Powell often had surprises for the boys, Hall said, and the younger boy had a sore foot that Hall thought he had banged into something, but police later determined that Powell had used a hatchet to chop at his sons' heads and necks.
"I'm saying, 'Let me in, Josh, let me in,'" Hall said. "I realized I didn't have my phone in my hand and I could smell gas. Too much time had passed and I could smell gas."
Hall said she went to move her car and call 911, whom she said did not acknowledge that it was a true emergency. She called her supervisor, but it was too late.
"I said to [her boss] Lynne, something terrible is happening here, and I was on the phone with Lynne when the house exploded," she said.
Powell had laced the home with gasoline and accelerants and set it on fire, blowing up the home. All three were killed.
"I wanted to get to the kids," she said. "I wanted to get to the kids. I would have broken in if I could."
Hall told the story of what happened over and over again to authorities as they arrived and assured her she could not have done anything differently. She blames Powell fully for what happened, and said she never thought he would hurt the boys. But now, she said, she knows he would have done anything to kill them, even if they had the supervised visitation somewhere else.
"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," Hall said. "It took just a second. When I close my eyes I see him and he was so normal. He did not look like a monster."
She has sweet memories of the boys.
"I loved the boys. I was like a grandma to the boys. They crawled all over me," she said with a laugh.
Braden, who was just 5, "was a free spirit" who "smiled all the time."
"He was always making things and giving them to me," she said.
The older boy "was smart and funny. He loved bugs and frogs. He had a bird." Charlie was looking forward to having his own frog pond when he moved back with his dad, she said.
"They are not going to grow up," Hall said later. "They're not ever going to look at bugs and frogs again."
Hall said she loves working with children. "God called me to do this," she said.
"I am not going to be his victim. He's not going to destroy my passion for children. He's not going toss top me from working with children," she said.
But the memory of Charlie and Braden will stay with her forever, Hall said.
"The world lost two beautiful boys to a monster."
Copyright 2012 ABC News Radio