Powell had grabbed the two young boys from a social worker who was there to monitor visitations on Sunday. He forced his sons inside the house and locked the door.
The child services worker pleaded with a 911 operator to get police to the home.
Caller: "I think I need help right away. This is the craziest thing- He looked right at me, and closed the door...I smell gasoline, and he won't let me in."
Dispatcher: "Ma'am, they have to respond to emergency, life-threatening situations first."
Caller: "This could be life-threatening. I am afraid for their lives."
It is now known that at the time Powell was attacking his sons with a hatchet, right before using 10 gallons of gasoline to ignite the home.
Caller: "He blew up the house and the kids! They are 5 and 7- they were one step ahead of me- and then he slammed the door in my face."
Firefighters arrived at the scene in three minutes. A flood of calls from neighbors followed the explosion.
One of the callers was Powell's sister Alina, who had just received a final voice mail from her brother saying, "I am not able to live without my sons. I'm sorry to everyone I've hurt. Goodbye."
"I'm terrified to drive over there," said Powell's sister Alina, audibly crying. "I'm afraid of seeing something I don't want to see."
Josh Powell and his two sons died in the explosion. He was declared a person of interest in the 2009 disappearance of his wife Susan in Utah.
On Tuesday a Utah prosecutor said that he's now convinced that Susan Powell is dead; her body has never been found. Had her death been ruled a homicide prior to Sunday's murder-suicide, Josh Powell most likely would not have been allowed visitation rights with his sons.
ABC News contributed to this story.