"The fact of the matter is we didn't have much control or any control when dealing with a foreign government," Lacy said. "Hindsight is 20/20."
Lacy said she would not change a thing about how she approached the investigation, except perhaps making a stronger effort to keep details from the media. She said Karr's graphic, detailed claims and his growing fascination with at least one girl in Thailand forced her hand.
"We had probable cause to arrest him," she said. "We also had taken advantage of a forensic psychologist ... who advised that this person was dangerous, this person was escalating."
She added: "We felt we could not ignore this, we had to follow it."
Lacy also said it is difficult to verify any suspect's claims of killing JonBenet Ramsey because every bit of physical evidence in the case has been disclosed to the public over the past 10 years. She noted that several suspects have been investigated in the recent past.
"This guy confessed on numerous occasions in great deal," said Peter Maguire, a deputy district attorney. "He confessed in e-mails, he confessed in telephone conversations ... he admitted it to a police officer. This was a bizarre crime and the person who committed this crime acted in a bizarre way."
JonBenet's father found the little girl's 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, who in e-mails and telephone conversations expressed a fascination with JonBenet and Polly Klaas, a murdered California girl, said after his arrest in Thailand this month that he was with JonBenet at the time of her slaying, which he called an accident.
But DNA tests did not match Karr to material found on the girl's body. Prosecutors suggested in court papers that Karr was just a man with a twisted obsession who confessed to a crime he did not commit.
"John Karr inserted himself into this," Lacy said. "I don't have a great deal of sympathy for John Karr."
Wexford Karr, the father of John Mark Karr, had no comment for reporters as he left a courthouse in Boulder Colorado Tuesday, CBS News Denver affiliate KCNC reports.
Lacy has been sharply criticized from the governor and others for detaining and arresting Karr. She estimated costs in the Karr investigation were at least $9,300, and told reporters she has received citizen calls calling for her to be "tarred and feathered" and "run out of town."
"The decisions were mine," Lacy said. "The responsibility is mine and I should be held accountable for all decisions in this case."
Lacy, whose statement was briefly interrupted by a fire alarm at the courthouse, also said investigators surreptitiously took DNA samples from Karr from multiple sites before he was detained. But Karr balked at providing a mouth-swab DNA test, considered the best way to confirm his involvement.
"We couldn't get his consent because he didn't know he was under investigation and we couldn't alert him at that time," she said.
CBS News legal analyst Andrew Cohen said Karr was not the first person to have been checked out and then dismissed as a subject. But the Karr episode generated new leads.
"The investigation into Karr's involvement cost about $15,000, none of which was in addition to Boulder's existing budgets. We have more answers now than we did before," Cohen said. "In the end, it came down to judgment. In the opinion of investigators, Karr was someone worth taking a closer look at. Boulder officials did that and came up empty — an exercise they have gone through many times over the past 10 years. The big difference here is that we all knew about it."
Lacy also said Tuesday that Karr made some "pretty bizarre" claims in hundreds of pages of e-mail conversations with University of Colorado journalism professor Michael Tracey, who alerted authorities to Karr's claims.
But Karr just might have been confessing to an obsession instead of a killing, CBS News correspondent Kelly Cobiella reports. That obsession with JonBenet Ramsey's life and the 6-year-old's death was detailed in four years of e-mails to Tracey.
Lacy conceded authorities were not able to confirm that Karr was in Boulder at the time of the slaying. Investigators said they felt sure of his whereabouts until Dec. 23, 1996, three days before the girl's body was found.
Karr's father and brother, Wexford and Nate Karr, went to the Boulder jail on Tuesday, but left without commenting.
JonBenet's aunt, Pamela Paugh, said she was not terribly shocked to learn that Karr's public confession to being with JonBenet when she died did not hold up.
"I think one of two things is going on: Either he is quite disturbed, and in that respect needs a lot of help and care, or he has perpetrated quite the fraud on the American public and the victims' families, and he needs help and a lot of care," Paugh told CBS' "The Early Show."
"For us, this isn't a story that ebbs and flows — this is real life. So we have to maintain a certain amount of calmness through anything that happens. Otherwise you can be on a rollercoaster every week of your life."
DA investigator Tom Bennett said the Karr case had triggered new leads, though he said the information typically provided is of little value. Lacy said some 200 people have been investigated in the case over the years.
Karr had made graphic claims in a series of e-mails about the 6-year-old's killing, describing sexual acts with her, her death and writing at one point that he envisioned Johnny Depp playing him in a movie about how he killed JonBenet. He wrote that the movie would make $1 billion.
Karr was never formally charged in the slaying.
Lacy said Karr emerged as a suspect in April after he spent several years exchanging e-mails and 11 telephone calls with Tracey, who had produced documentaries on the case.
The District Attorney's office released explicit details of statements Karr had made in those exchanges with Tracey, who had 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.
The backlash was swift. Defense attorney Seth Temin said Karr never should have been arrested. Republican Governor Bill Owens said Lacy, a Democrat, "should be held accountable for the most extravagant and expensive DNA test in Colorado history."
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.
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.