John Mark Karr – the creep who became famous after falsely confessing to killing JonBenet Ramsey – is still obsessed with her unsolved murder and is undergoing a sex change!

In an exclusive ENQUIRER interview, Karr’s former fiancee reveals the 45-year-old sicko regularly visited JonBenet’s grave site and now lives as a woman.

"John is a deeply troubled individual," Brook Simmons, 25, told The ENQUIRER.

In August 2006, the former teacher and alleged pedophile made worldwide headlines when he falsely confessed to slaying JonBenet on Dec. 25, 1996, in Boulder, Colo.

Police tracked down Karr in Bangkok, Thailand, and arrested him. He was flown to the U.S. by Boulder detectives, but DNA tests failed to place Karr at the scene of JonBenet’s murder, and he was never charged.

He moved to Alabama to live with his father, and met Brook, who had a 3-year-old daughter from a previous relationship.

"At the time we lived about 15 miles from where JonBenet is buried in Marietta, Ga. John went to visit her grave at least once a week during the time we dated," Brook told The ENQUIRER.

"I went to the cemetery with him a few times and was surprised at how emotional he got. At times John would start to tear up. He’d say, ‘I took her life and because of me, she isn’t here. It’s my job to feel remorse for her.’

"One time, John wanted me to take a picture of him sitting near her gravestone. I thought it was strange, but I loved him, so I did it.

"Occasionally, he’d bring flowers and place them on JonBenet’s grave. Once he left a ceramic angel figurine there."

Brook says she broke off their engagement due to physical and emotional abuse, and while she believes Karr has returned to Thailand, he changed his name before leaving the U.S.

The ENQUIRER has obtained court documents filed in King County District Court in Seattle, Wash., that indicate Karr legally changed his name to "Delia Alexis Reich." He later submitted another name change to the court, asking to be called "Alexis Valoran Reich."

Karr claimed he started the sex change process in Bangkok in 2006, according to Brook.

"John told me that if we ever broke up, he’d continue the process of changing into a woman," she divulged. "He said that he was a woman trapped in a man’s body, and only quit the transformation after we started to date.

"I’m just glad I never introduced my little girl to him."