Boulder County District Attorney Mary Lacy said Karr believes he killed the girl despite the fact that "he is not the killer," CNN reported Wednesday.
Although DNA evidence from JonBenet's clothing proved Karr's claims to be false, Lacy said his statements initially seemed credible.
"Hindsight is 20/20," Lacy said. "After the game is over, it's easy to criticize."
The prosecutor said she takes full responsibility for Karr's arrest in Thailand.
"The decisions were mine. The responsibility was mine. I should be held accountable for all decisions made in this case."
She said Karr had posed a threat to other children.
"I can't say there is evidence that he was involved in another crime, but he was paying attention to a 5-year-old girl in school. That was a turning point for us. He was engaging in grooming behavior."
Karr was being held in a Boulder jail without bail awaiting extradition to California on child pornography charges.