Los Angeles police detained Karr on a warrant from Boulder County, Colorado, after he arrived in California from Bangkok, Thailand, and turned him over to the county Sheriff's Department late Sunday, said sheriff's spokesman Steve Whitmore.
CBS Radio News correspondent Steve Futterman reports that a media circus developed at the airport, with reporters from both inside and outside the United States gathering. Outside the international terminal more than a dozen satellite trucks were waiting for Karr's arrival.
A helicopter brought him from the airport to the sheriff's Twin Towers jail shortly before midnight in a sobering end to a day that began in Bangkok and included champagne and fine dining aboard a Thai Airways flight.
"He is going to be housed here in the men's jail, kept in isolation in a 6-by-9-foot room with a bed, a toilet no windows and no phones," Whitmore said. "He'll get regular food. He'll get jail chow, he won't get king crab, I'll tell you that."
Karr was to be held in a "high power" cell for noteworthy inmates, checked by guards every 15 minutes, and separated from other inmates who often target suspected child molesters, sheriff's officials said.
Boulder County authorities said Karr was expected to have an extradition hearing in Los Angeles within days, and would be taken to Colorado if he waived extradition. No hearing date was immediately set.
Karr, who last week suddenly emerged as a suspect in a case long believed to have gone cold, told reporters in Thailand that he was with 6-year-old JonBenet when she died in the basement of her home on Dec. 26, 1996, but that her death was an accident.
U.S. officials have been silent about what Karr told them during interrogations.
The 41-year-old school teacher's return to the United States was voluntary, and he was not handcuffed before or during the 15-hour flight.
Dressed in a red, short-sleeve, button-down shirt and black tie, Karr was whisked through Don Muang International Airport in Bangkok. He chatted with fellow passengers at the departure gate.
Aboard the jet he took a window seat next to Mark Spray, an investigator with the Boulder County district attorney's office. The escort also included a U.S. Embassy official and an agent with "Homeland Security" on his shirt.