Defence lawyers for John Karr, a 41-year-old teacher repatriated from Bangkok last week to face charges over the killing, announced last night that he would not be charged with JonBenet's murder.

Instead of holding the key to the decade-old murder mystery, Mr Karr appeared instead to be a deluded fantasist after a DNA test failed to connect him with the crime.

Mary Lacy, the Boulder Country District Attorney, confirmed the decision. "The DNA associated with the victim in this case does not match John Mark Karr. The family of Mr Karr co-operated by providing circumstantial evidence that he spent Christmas with his family in Atlanta, Georgia, at the time of the 1996 murder," she said.

The news was greeted with derision by Bill Owens, the Republican Governor of Colorado.

"I find it incredible that Boulder authorities wasted thousands of taxpayer dollars to bring Karr to Colorado given such a lack of evidence," Mr Owens said. "Mary Lacy should be held accountable for the most extravagant and expensive DNA test in Colorado history."

Hair and saliva were taken from Mr Karr after his arrival from Thailand last week and tested over the weekend.

The decision of the Boulder County District Attorney to drop the arrest warrant against Mr Karr came about an hour before he had been scheduled to make his first Colorado court appearance. Hair and saliva were taken from Mr Karr after his arrival from Thailand last week and tested over the weekend.

That hearing was immediately cancelled and Mr Karr will now be extradited to California to face child pornography charges.

The decision is the latest twist in a case that gripped America for months after John Ramsey, the father of JonBenet, found her battered body in the basement on December 26, 1996, about seven hours after her mother stumbled on a bizarre letter claiming that she had been kidnapped and demanding a $118,000 ransom.

There were no obvious signs of a break-in or forced entry and the parents of JonBenet were treated as suspects but never indicted.

The mother of JonBenet, who knew about the FBI investigation into Mr Karr, died of cancer in June aged 49.

Mr Karr was arrested in Bangkok this month by Thai police working with FBI agents. Before his arrest he had admitted killing the six-year-old girl from Boulder, Colorado, in e-mails to a journalism professor who had produced three documentaries on the case.

Mr Karr, who has a criminal record in California for possessing child pornography, confirmed the content of his e-mails with a statement to reporters in Bangkok. "I was with JonBenet when she died," he said. "Her death was an accident . . . I loved JonBenet."

But almost immediately doubt was thrown on his "confession" after inconsistencies appeared in his version of events. It is not known how he could have been in Colorado at the time of the murder in 1996, or even known the Ramsey family, who say that they did not know him.

Seth Temin, Mr Karr's publicly-appointed defence lawyer, said: "We’re deeply distressed by the fact that they took this man and dragged him here from Bangkok with no forensic evidence confirming the allegations against him and no independent factors leading to a presumption that he did anything wrong."

The DNA found on the girl’s underwear was identified as belonging to a white male but has never been matched to a suspect. JonBenet had been strangled with a garotte made from cord and a paint brush, her skull fractured and her mouth duct-taped. Forensic evidence suggested she had been sexually assaulted.

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