BOULDER, Colo. — Prosecutors abruptly dropped their case yesterday against John Mark Karr in the slaying of JonBenet Ramsey, saying DNA tests failed to put him at the crime scene despite his insistence that he sexually assaulted and strangled the 6-year-old beauty queen.

Just a week and a half after Mr. Karr’s arrest in Thailand was seen as a remarkable break in the sensational, decade-old case, prosecutors suggested in court papers that he was just a man with a twisted fascination with Jon-Benet who confessed to a crime he did not commit.

“The people would not be able to establish that Mr. Karr committed this crime despite his repeated insistence that he did,” District Attorney Mary Lacy said in court papers.

The 41-year-old schoolteacher will be kept in jail in Boulder until he can be sent to Sonoma County, Calif., to face child pornography charges dating to 2001.

The district attorney vowed to keep pursuing leads in JonBenet’s death: “This case is not closed.”

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

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

According to court papers, Mr. Karr told the professor he accidentally killed JonBenet during sex and that he tasted her blood after he injured her vaginally. But the Denver crime lab conducted DNA tests last Friday on a cheek swab taken from Mr. Karr and were unable to connect him to the crime.

“This information is critical because … if Mr. 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,” Ms. Lacy said in court papers.

She also said authorities found no evidence Mr. Karr was in Boulder at the time of the slaying. She said Mr. Karr’s family provided “strong circumstantial support” for their belief that he was with them in Georgia, celebrating the Christmas holidays. JonBenet was found beaten and strangled at her Boulder home on December 26, 1996.

The defense attorney, Seth Temin, expressed outrage that Mr. Karr was even arrested.

“We’re deeply distressed by the fact that they took this man and dragged him here from Bangkok, Thailand, with no forensic evidence confirming the allegations against him and no independent factors leading to a presumption he did anything wrong,” Mr. Temin said.

In an interview yesterday with MSNBC, Gary Harris, who had been spokesman for the Karr family, said he knew the DNA would not match.

Mr. Karr has been “obsessed with this case for a long time. He may have some personality problems, but he’s not a killer,” Mr. Harris said. “He obsesses. He wanted to be a rock star one time. … He’s the kind of guy who wants to be famous.”

Earlier this month, Ramsey family attorney Lin Wood pronounced Mr. Karr’s arrest vindication for JonBenet’s parents, John and Patsy Ramsey, who had long been suspected in the killing.

Yesterday, 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.”

Mr. Wood said he still has great confidence in the district attorney. Patsy Ramsey died of cancer in June. JonBenet Ramsey’s aunt, Pamela Paugh, said she was disappointed that no one would be prosecuted in the case but added: “I think our justice system worked as it was supposed to.”

“We asked the DA to do her thing. She did it,” Ms. Paugh, who is Patsy Ramsey’s sister, said. “My disappointment came about the end of December 1996 when we didn’t have the killer then.”

Nate Karr, John Karr’s brother, said he was elated by the news. “We’re just going to be celebrating with family,” he said.

A Denver attorney who has followed the case from the beginning, Scott Robinson, said prosecutors may now be back at square one in the JonBenet case. He said Mr. Karr may be charged with lying about his role in the case.