Josh Powell's computer had images of animated incestuous sex on it, authorities have revealed.
The images were realistic computer-generated depictions of parent-child sex, Detective Ed Troyer of the Pierce County Sheriff's Department told the Associated Press.
The computer was taken by police following the 2009 disappearance of Powell's wife, Susan Cox, but the images were brought up for the first time publicly in a court custody hearing last week.
The court ruled that because of images that were "specifically related to the children's welfare" on his computer, Powell would have to undergo an extensive psychosexual evaluation, polygraph test, and counseling before he could regain custody of the boys. The court would not disclose what the images depicted.
Following the custody hearing, Powell told ABC News that he had no idea what kind of images the West Valley City, Utah, police had found on his computer that would factor into a custody hearing. He said he could not remember having any inappropriate images on his computer, and said it was possible someone had emailed the images to him.
But four days later, Powell killed his children -- Charles, 7, and Braden, 5 -- attacking them with a hatchet and then blowing up his house with himself and his sons inside.
Steve Downing, the attorney representing Cox's parents, Chuck and Judy Cox, said if he had known the nature of the images that were found on Powell's computer, he would have asked the court to change the terms of custody.
Powell originally lost custody when his father, Steve Powell, with whom Powell and his sons lived, was arrested on child pornography and voyeurism charges in September.
He had moved in with his father after his wife Susan Cox disappeared. Powell was the lone "person of interest" in her disappearance.
The long drama surrounding Powell came to an explosive end Sunday during a supervised visit at his home. Powell welcomed the children inside and locked out the social worker.
He could be heard telling son Charlie he had "a big surprise" for him. The social worker then heard Braden cry out. And minutes later the house exploded.
Powell sent emails and left voicemails for relatives and friends minutes before the explosion saying he could not live without his boys any longer and had to protect them.
New information emerged today on how the supervised visits had been switched from public places to Powell's home.
"We had been having supervised visits, and there were no concerns raised during those visits," Sharon Gilbert, an official at the Department of Social and Health Services, told ABC News' "20/20."
Because of Powell's notoriety in the case about his missing wife, officials decided to move the visits to his home.
"Josh Powell was recognizable, as were the children. And that could also be disruptive to visits that are occurring with other families," Gilbert said of the decision.
Both Gilbert and social worker Elizabeth Griffin-Hall said they did not see any warning signs that Powell would eventually lure the children into his home to kill them.
"How this happened is that Josh Powell was really, really evil, Griffin-Hall said. "I couldn't have stopped him."
Joseph Rhee contributed to this report.
Learn more on "Sins of the Father," tonight on "20/20" at 10 p.m. ET.