Garnett, 52, a Democrat, has no opposition in either party for the November election. He would replace Mary Lacy, who must step down in January because of term limits.
What changes he might bring, if any, to the investigation are not clear.
"I intend to treat the case like any other case, in that the DA's job is to take a very thoughtful, careful and clear-eyed view of the evidence and what can be proven in court," he said.
JonBenet, a 6-year-old beauty queen, was found dead in the basement of her parents' Boulder home just after Christmas in 1996. Since then the investigation has undergone a series of jarring twists and public embarrassments but has never led to a trial or a conviction.
Under Alex Hunter, the district attorney at the time, the investigation seemed to go nowhere. Hunter said John and Patsy Ramsey were under an "umbrella of suspicion," but a grand jury returned no indictments. Lou Smit, an investigator hired by Hunter for the case, resigned and proclaimed John and Patsy Ramsey innocent.
Lacy was elected in 2000, after Hunter decided not to seek another term. She took the investigation away from police in 2002, promising to look at the evidence with "fresh eyes."
By 2003, she began to poke holes in Hunter's umbrella of suspicion. She publicly agreed with a judge in a related defamation case who said the evidence pointed more toward an intruder than a family member as the killer.
Lacy's tenure saw perhaps the most bizarre chapter in the investigation, the 2006 arrest and subsequent release of John Mark Karr. He claimed he was with JonBenet when she died, but DNA tests failed to put him at the crime scene.
The case took a significant step forward last week when Lacy announced that male DNA found on JonBenet's clothes almost certainly came from her killer and that the killer was an outsider.
Lacy publicly exonerated JonBenet's family and said she was sorry for the years of suspicion they endured.
Patsy Ramsey did not live to hear the apology; she died of cancer in 2006.
Authorities say the DNA doesn't match any profiles in state or national databases. But both databases are growing, and investigators say a match may be found one day.
A former FBI criminal profiler, Gregg McCrary, said the impending handing over of the investigation to another DA could be a good thing.
"One of the reasons cases sometimes remain unsolved is because a DA or an investigator has a particular hypothesis of who did it," he said. "So it can be helpful to get fresh eyes on the case, assuming they're not coming in with a preconceived idea."
L. Lin Wood, an attorney for the Ramseys, criticized the police but praised Lacy.
"I can tell you the differences are very simple. The Boulder (Police) Department had investigators who had no homicide experience," he said.
In contrast, he said, Lacy and Smit "took the proper approach," focusing on the DNA and following the evidence.
Wood said he hopes the next district attorney will take the same approach. But "short of a dramatic new lead," there's not much to do but wait for a DNA match, he said.
Garnett, a former Denver deputy district attorney who has been specializing in commercial civil cases, said the topic of JonBenet still comes up frequently as he campaigns, but he said public fatigue is setting in.
"Most people in Boulder County are tired of the case," Garnett said. "I had one person at a political event say 'The people in Boulder County don't ever want to hear about this case until you've solved it."
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