The parents of missing mom Susan Cox-Powell spoke out Wednesday for the first time since chilling 911 calls were released from the fire that killed their grandsons.

In a primetime exclusive interview with HLN’s Nancy Grace, Susan’s father said his first reaction when he heard from a friend that his daughter’s two young boys were killed in a fire apparently set by their own father was disbelief.

“It’s just so outlandish and hard to believe that somebody would actually set their house on fire or set off an explosion and kill two innocent children,” Chuck Cox recalled.

Even after seeing what was left of Josh Powell’s home surrounded by police tape in the wake of the explosion, Cox did not want to accept it.

“They can’t be gone. Because nobody would do that. It couldn’t happen. So I just couldn’t fathom it,” he said.

Cox and his wife Judy told Grace that 7-year-old Charlie and 5-year-old Braden seemed to grow more willing to talk about their missing mother since they were taken from Powell’s custody last fall. At first, though, they tried to evade questions about her, as Cox believed they were instructed to do by their father.

“All they would say was the last time they saw mommy was on a camping trip…Then they would go back to, ‘I don’t know, I don’t remember’…a trained and coached response,” he said.

Josh Powell was a suspect in Susan Cox-Powell’s December 2009 disappearance, but he was never charged. He told police that he took their two sons camping in the middle of the night and she was gone when they returned.

Chuck Cox stated that he now has no doubt that Josh Powell killed his daughter.

“I know he’s responsible,” Cox said.

Police believe Josh Powell attacked his two sons with a hatchet in his Washington home Sunday after locking out a social worker who was supposed to supervise their court-ordered visitation. They say he then set the house on fire, causing an explosion that killed him and the boys.

“This could be life-threatening…I’m afraid for their lives,” the social worker told a dispatcher in a 911 call moments before the house exploded.

For the full interview with Chuck and Judy Cox, watch “Nancy Grace” Wednesday at 8 p.m. and 10 p.m. EST on HLN.