The sudden decision to drop the case against John Mark Karr in the slaying of JonBenet Ramsey snuffed out yet another lead in the enduring mystery of who killed the 6-year-old beauty queen, but prosecutors vowed that the investigation would go on.

"This case is not closed," said District Attorney Mary Lacy, who planned to further explain her decision at a news conference Tuesday.

Read the district attorney's Motion to Quash (People v. Karr)

The case has never been closed, not since JonBenet's father found her body in the basement of their Boulder home on the day after Christmas 1996. For years, suspicion has focused on either an intruder or the girl's parents, John and Patsy Ramsey.

Karr, a 41-year-old schoolteacher fascinated with JonBenet and Polly Klaas, a murdered California girl, said after his arrest in Thailand earlier this month that he was with JonBenet at the time of her slaying, which he called an accident.

But Lacy said DNA tests and investigators did not put Karr at the crime scene. Prosecutors suggested in court papers that he was just a man with a twisted obsession who confessed to a crime he didn't commit.

Karr was being held at the Boulder jail until he can be sent to Sonoma County, Calif., to face misdemeanor child pornography charges dating to 2001. An extradition hearing was scheduled for Tuesday afternoon.

Karr was never formally charged in the slaying. In court papers, Lacy defended the decision to arrest him and bring him back to the United States for further investigation, saying he might have otherwise fled and may have been targeting children in Thailand.

Lacy said Karr emerged as a suspect in April after he spent several years exchanging e-mails and 11 telephone calls with a University of Colorado journalism professor who had produced documentaries on the case.

• Timeline: The JonBenet Ramsey Murder Investigation

The DA's office provided explicit details of Karr's statements to professor Michael Tracey, who alerted authorities. Karr told the professor he accidentally killed JonBenet during sex and tasted her blood after he injured her, prosecutors said.

"Are you asking me why I killed JonBenet? I don't see it that way," Karr wrote in a May 22 e-mail. "Her and I were engaging in a romantic and very sexual interaction. It went bad and it was my fault."

But the claims were lies, prosecutors said. The Denver crime lab conducted DNA tests Friday on a cheek swab taken from Karr and were unable to connect him to the crime.

"This information is critical because ... if Mr. Karr's account of his sexual involvement with the victim were accurate, it would have been highly likely that his saliva would have been mixed with the blood in the underwear," Lacy said in court papers.

She also said authorities found no evidence Karr was in Boulder at the time of the slaying. She said Karr's family provided "strong circumstantial support" for their belief that he was with them in Georgia, celebrating the holidays.

Defense attorney Seth Temin said Karr never should have been arrested.

When Karr was arrested in Thailand, Ramsey family attorney Lin Wood pronounced it a vindication for JonBenet's parents. Patsy Ramsey died of cancer in June.

On Monday, the attorney said: "From day one, John Ramsey publicly stated that he did not want the public or the media to jump to judgment. He did not want the public or the media to engage in speculation, that he wanted the justice system to take its course."

Nate Karr, John Karr's brother, said he was elated his brother would not be charged. "We're just going to be celebrating with family," he said.

But Scott Robinson, a Denver attorney who has followed the case from the beginning, said Karr may be charged with lying about his role.

"Seems to me there should be some criminal consequences," he said. "He has cost the taxpayers an enormous amount of money."

The district attorney defended the handling of the case, saying there was no way to take a cheek swab from Karr without alerting him that he was under investigation.

Also, Karr was about to start a teaching job in Thailand, and in his correspondence began to describe an interest in several girls "in much the same terms that he had described his interest in JonBenet," Lacy said in court papers.

In a July 19 e-mail, Karr described feeling excited because two 5-year-olds were "flashing their hot little bellybuttons at me" and later said a "naked little foot felt so sexy in my hand," prosecutors said. Karr's arrest was less than a month later.

More coverage available in FOXNews.com's Crime Center.