Without it, experts say, it is still possible — but much more difficult — to build a strong murder case against the 41-year-old globe-trotting teacher who has said he was there when the girl died 10 years ago, but stopped short of an outright confession.
Karr's first court appearance is scheduled for Monday, an advisement hearing expected to last only a few minutes.
One of District Attorney Mary Lacy's strengths could be Karr himself, who chose not to fight extradition to Colorado after telling reporters he was there when JonBenet was killed.
Authorities in Sonoma County, California, who arrested Karr for possession of child pornography in 2001, said he had made "uncertain allusions to placing himself in the killer's role" in talking about JonBenet and Polly Klaas, slain in 1993 in Petaluma, California.
"From everything I've seen this guy wants to come back to Boulder," said former Denver prosecutor Craig Silverman. "The only question is whether he's coming to Boulder for the first time. I suspect Mary Lacy has some evidence that it's not his first visit."
JonBenet's parents, John and Patsy Ramsey, were initial targets of an investigation that included a grand jury's decision not to indict anyone. Patsy Ramsey died in June after learning authorities had turned their attention to Karr, who was living in Thailand when he was detained earlier this month.
In a court filing, prosecutors said they have evidence that has not been disclosed despite 10 years of public scrutiny of the case.
Investigators have said DNA from a white male was found in blood spots in JonBenet's underwear, but a Ramsey family attorney said two years ago it did not match any of the 1.5 million samples in an FBI database at the time. Other physical evidence includes a ransom note, a boot print found outside the Ramsey house and some indications an intruder could have entered through a basement window.
But mistakes were made early in the investigation. John Ramsey and others were allowed to roam the house — the crime scene — before his daughter's body was found. Friends of the family came to the home, called by the Ramseys after they found the ransom note.
All of this could come back to haunt prosecutors, experts said.
"You need to overcome a lot of things; in order to convict Karr or anyone else, you need to overcome the evidence that seemingly could be interpreted as pointing at the Ramseys and the severely compromised crime scene," said Scott Robinson, a Denver attorney familiar with the case. "If Karr's DNA is linked to the scene, it's all over but the verdict.
"But if in fact there is no DNA match, a defense attorney could argue that the confession means nothing."
Prosecutors can overcome police mistakes with solid work, said Bob Grant, a former district attorney who served as an adviser on the case in the 1990s.
"I've never seen a case in trial where the defense didn't find something to attack the investigation about in terms of technical investigative techniques, and I've never seen an investigation that was perfect," Grant said. "If there's DNA able to be matched to him, then the mistakes are of little or no consequence."
After Karr was detained in Bangkok, Lacy told a news conference that sometimes it becomes necessary to make an arrest before an investigation is complete and that much work remained in the Karr investigation. Since then, prosecutors have asked a judge to keep the arrest warrant affidavit sealed for at least two weeks while they work on a case still in its "very early stages."
"The press conference was not to announce `We got our man,' it was a press conference to manage expectations of the public," said Norm Early, a former Denver district attorney.
He and other experts said Lacy deserves the benefit of the doubt and time to develop her case.
"I presume competence on the part of prosecutors, but that presumption has been put to the test by the Boulder DA's office in this case in the past," Silverman said. "I just can't believe they would have done something without something that made them sit up straight in their chairs and say `Oh my God, send a guy to Thailand."'