BOULDER - Colorado prosecutors have defended their arrest of John Mark Karr for the notorious unsolved murder of 6-year-old JonBenet Ramsey, saying they were misled by the schoolteacher's confession.

Boulder County District Attorney Mary Lacy conceded that she had no evidence linking Karr to the 1996 crime aside from his bizarre emails to a college professor, but said authorities had to act because they considered him an imminent threat to a 5-year-old girl in Thailand.

"John Karr himself sincerely believes he killed JonBenet Ramsey so I have no sympathy for him," Lacy said.

"Because he believed it himself and continues to believe it his (confession) had all of the emotional impact that you would expect (from the killer)."

Associated Press reported today Karr told University of Colorado professor Michael Tracey by email Johnny Depp could play him in a movie which he thought would make $US1 billion ($NZ1.58 billion). He explained that he liked little girls as long as they were no older than 10.

Karr reportedly said in the emails he had killed JonBenet with a flashlight after the strangulation and that he carried her, covering her with a blanket he put over his left shoulder.

Lacy said she was now convinced that Karr, 41, was not JonBenet's killer and his confession was all a lie, leaving authorities with no suspects in the mysterious, decade-old crime despite having "seriously" investigated some 200 people.

Karr was arrested in Bangkok two weeks ago for JonBenet's murder, triggering a media frenzy that raged until he was abruptly dropped as a suspect yesterdat, after DNA tests failed to link him to blood mixed with saliva found in the girl's underwear.

Boulder prosecutors have come under withering criticism for their decision to extradite Karr, even though they could not prove that he had ever been in Boulder and had no forensic evidence tying him to the crime scene. Karr lived in Alabama at the time of the murder.

'Every parent knows his name'

Lacy and her top deputy, Peter Maguire, said they surreptitiously obtained samples of Karr's DNA in Bangkok while he was under surveillance, but found them unreliable and felt pressure to arrest the teacher after he became enamoured with a 5-year-old girl in one of his classes.

"He was expressing feelings towards this child in the same way that he was expressing feelings towards the dead child (JonBenet)," Lacy said.

The DNA, which tests show was likely the saliva of a white male mixed with JonBenet's blood, has never been matched to a suspect in the brutal murder. Members of JonBenet's family have been excluded as having left the DNA.

Karr's detailed confession included an explanation of why his saliva might have been found mixed with JonBenet's blood in her underwear, which prosecutors said lent credibility to his confession.

The prosecutors said they remain concerned about Karr's apparent attraction to young girls. Karr remains in custody and faces extradition to California to face child pornography charges, and "at least every parent in this country has seen his face and knows his name," Lacy said.

She said she has a "very high level of frustration" that JonBenet's murder was still unsolved since the child beauty queen's battered body was discovered, partially covered by a blanket, in the basement of her Boulder home on December 26, 1996.

JonBenet was found about seven hours after her mother came across a bizarre letter claiming the former Little Miss Colorado had been kidnapped and demanding $118,000 in ransom. She had been strangled with a homemade garrote, her skull fractured and her mouth duct-taped.

Lacy said investigators remained "baffled" by the case, which has long fascinated Americans.

- REUTERS