"When you look at these cases, 99 percent are released relatively unharmed," Lanning said. "Most people fall off their chairs when they hear that."
"Typically, they stop a child and lure them into a car, or the woods or backyard, or a basement and do something to the child -- sexual perhaps," he said. "Then, they let them go and the child is home before they knew they were missing."
But in cases of so-called "long-term" abductions such as that of Patz and Adam Walsh, who disappeared in a Florida shopping mall in 1981, "the outcome is not so good," Lanning said.
He added that investigators can't make assumptions based on other similar crimes.
"We don't have a huge number of these cases, and many are unknown," he said. "They have to consider all possibilities and they can't put all their eggs in one basket."
"The key here is consistency," said Lanning. "Is what he says he did consistent with what we know about him? If he says nothing went on sexually and then they find out this guy had been grooming and seducing 6-, 7-, 8-year-old boys, there's an inconsistency."
Police may holding back some of the details of the confession -- even hints of sexual molestation -- to spare Etan's parents, Stan and Julie Patz.
"They might not go into it, especially in a case this old," said Lanning. "Imagine the poor mother and father go through this 33 years later. He could have told them he did something with the boy and the police aren't saying it."
But investigators cannot rule out the possibility that Hernandez molested the boy, according to Lanning.
"It's easier for most of these guys to rationalize killing a child than having sex with a child," he said.
An FBI study of 500 children who were abducted and murdered found that about 75 percent of them had been killed within three hours of abduction. Some abductors kill for sadistic reasons, others for sexual gratification, but there are also those who kill because they "screwed up," according to Lanning.
"One of the least likely things a sexual predator will do is kidnap and murder his victims," he said. "A smart sexual predator doesn't kidnap anyone."
But sometimes things go wrong and the child "starts screaming and yelling and kicking and biting," said Lanning. "Now, he's got to stop the kid."
A certain number worry afterwards that if they release the child, they will tell, and so they go on to suffocate the child.
But all this is speculation, according to Lanning.
"What you think happened," he said, "is not so important as what you can prove happened."
On Friday, Hernandez was on a suicide watch at a New York City hospital just hours before he was arraigned on second-degree murder charges.
Russell Goldman and Richard Esposito contributed to this report.