Elizabeth Griffin-Hall is the social worker who was there when Josh Powell blew up his home with his two sons inside.
Now, Griffin-Hall is telling her harrowing story to ABC's Chris Cuomo, airing on 20/20 Friday.
She spoke about delivering little Charlie and Braden to the house for a scheduled supervised visit.
She recounted, "As the door closed he said, 'Charlie, I've got a big surprise for you' and then I heard Braden cry out and so I knocked and I knocked and I rang the doorbell and I started yelling, 'Josh let me in!'"
It was then that Powell attacked his sons with an ax before blowing up the house.
She talked about her frantic efforts to get help from a 911 operator.
Griffin-Hall: "How long will it be?"
911: "I don't know, ma'am. They have to respond to emergency life-threatening situations first."
Griffin-Hall: "This could be life-threatening. I'm afraid for their lives."
"I'm not getting an emergency response. They're not responding to me at the level that I'm telling them, that I can smell the gas. I can smell the gas," Griffin-Hall told Cuomo.
INSIDE EDITION's Diane McInerney spoke with Chris Cuomo.
McInerney asked, "Does she [Griffin-Hall] believe there is anything that she could have done to prevent this tragedy?"
Cuomo said, "Yes she does. She believes very deeply that she could have prevented it, that she should have stopped it. She blames herself. There is no basis in fact for that though. This is just the guilt of a woman who loved so deeply for these kids and the others that she takes care of, who is so confused and shocked by what she saw in Josh Powell. Someone who she spent hours and hours with and thought he was one person and then to find out that he was actually what she calls 'This evil monster,' has put her in a horrible place emotionally."
We're also learning more about how Powell plotted the unspeakable act. A surveillance picture showed him withdrawing cash from an ATM the day before. He used the money to buy five-gallon gas cans to set the fire.
Cuomo asked Griffin-Hall, "What is the hardest thing for you to deal with emotionally when you think about losing your boys?"
She responded, "That they won't grow up. That they didn't get to see the sun the next day. All that life has in store was taken away from them."