Susan Powell's Parents: Josh Powell's Dad Knows What Happened To Missing Daughter

The parents of missing Utah mom Susan Powell lashed out at her father-in-law in a TV interview, telling Fox, "I'm sure he knows where she is or where his son put her."

Charles and Judy Cox, whose daughter went missing in December 2009, appeared on Fox News on Tuesday to discuss new evidence linking Josh Powell to his wife's case. "I'm convinced that his dad knew everything that was going on, if not helping with it," Charles Cox told anchor Martha MacCallum.

Josh Powell, the primary "person of interest" in the missing person case, killed himself and his two sons by setting fire to his home earlier this year. Despite evidence suggesting Josh played a role in Susan's disappearance, he was never arrested by authorities in West Valley City, Utah. As a result, Susan Powell's family believe the authorities are at least partially responsible for the death of her two sons, Braden, 5, and Charles, 7.

"Look what it did -- it killed my nephews, by not putting [Josh Powell] behind bars when he should have been," Denise Cox, Susan's sister, told ABC News. "They didn't act on anything that they should've acted on."

Even if an arrest was made, though, that doesn't mean Josh Powell would have stayed in jail, Susan Powell's father pointed out.

"If they did arrest him and he was convicted, then our grandchildren would be alive," Charles Cox said on Fox News. "But that's a big 'if' because arresting a person and convicting them are two separate things, especially with no body."

It now appears authorities had other evidence, though. Court documents unsealed on Friday revealed that Susan Powell left a letter in a safety deposit box saying she did not trust her husband and that if she were to die, "it may not be an accident, even if it looks like one," the Associated Press reported.

Authorities also found Susan Powell's blood on a floor next to a sofa, which appeared to have been recently cleaned and had two fans set up to dry it.

Josh Powell, who is described in the documents as "unwilling to help" the investigation, said he took his two sons on a late-night camping trip the night Susan Powell disappeared. Investigators found Susan's cell phone in her husband's car, which he "did not have an answer" to explain, the Associated Press reported.

Susan's purse, keys and credit cards were among some of the other belongings found in the couple's bedroom.

The documents also show that Susan Powell had several life insurance policies, totaling $1.5 million in value.

"There is direct evidence. There is circumstantial evidence. There is motive," Pierce County prosecutor Mark Lindquist told ABC News. "There is everything but the body."

The court documents also detail an apparent obsession Steven Powell had with his daughter-in-law. In a cabinet belonging to Steven Powell, authorities found several images of Susan, some of which showed her in her underwear, while others featured her face copied onto other women's naked bodies. Another image depicted Steven Powell masturbating to an image of his daughter-in-law.

According to Susan Powell's personal journals, she did not want anything to do with Steven Powell, whom she described as a pedophile.

With all of this evidence uncovered, Susan Powell's parents feel authorities had enough to arrest her husband.

"We depended on their professionalism and opinions and experience," Charles Cox told Fox News. "We don't know all the evidence they have. It appeared to us that he should have been arrested a long time ago."