Josh Powell reportedly told his children - Charles, 7, and Braden, 5 - that he had a 'big surprise' for them as he took them inside the house in Graham, about 25 miles southeast of Tacoma, only to butcher them with a hatchet. He also set the house on fire, killing himself on Sunday Feb 5.

The gripping details of the incident were revealed by social worker Elizabeth Griffin-Hall as she spoke to the ABC News on Thursday.

Powell had often planned surprises for the boys, Griffin-Hall recalled, but added that things were totally different on Sunday. He caught my eye, his shoulders were slumped. He had a sheepish look, Griffin-Hall told 20/20 on the ABC News. He just shrugged his shoulders and slammed the door, she added.

I'm saying, 'Let me in, Josh, let me in, Griffin-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, the social worker added.

She picked up her phone from her car and alerted 911, but the woman who spoke to her from the other end asked Griffin-Hall whether it was a true emergency. She then called up a higher official in the 911 response team but it was already too late.

I said to (her boss) Lyn, something terrible is happening here, and I was on the phone with Lyn when the house exploded, she said.

Griffin-Hall said she had been taking care of the boys for three months. The boys were staying with their grandparents since their father Powell was the sole suspect in his wife Susan Cox's disappearance in 2009. Powell had lost the custody of the children in September. He again filed an appeal in the court for the custody and lost the appeal to get the children back just four days before he decided to kill them.

I wanted to get to the kids, Griffin-Hall said. I wanted to get to the kids. I would have broken in if I could.

How this happened is that Josh Powell was really, really evil. I couldn't have stopped him, she said.

Griffin-Hall remembers the sweet boys

Braden was a 'free spirit' soul with a smile on his face always. Charlie was a smart and funny boy who loved bugs and frogs.The boy owned a bird, but he wanted to have his own frog pond.

They are not going to grow up, Griffin-Hall said. They're not ever going to look at bugs and frogs again. I loved the boys. I was like a grandma to the boys. They crawled all over me.

The world lost two beautiful boys to a monster, she added.