Ms. Lacy’s statement provided no details about what had led investigators to the suspect or why, if they had been on his trail for some time, they swooped down on him yesterday. Nor did she spell out what charges he would face, and it was not clear how soon he would be returned to this country. Ms. Lacy’s statement said she would not comment further until a news conference this morning, and calls to her office were not returned.

The suspect was identified by a law enforcement official, speaking on condition of anonymity because he was not authorized to discuss the case publicly, as John M. Karr. The Associated Press quoted a lawyer who represented the Ramseys in the 1990’s as saying Mr. Karr had lived in Conyers, Ga. The Ramseys lived in an Atlanta suburb, Dunwoody, about 30 miles away, before they moved to Colorado.

The Associated Press initially reported that Mr. Karr, 42, was already in custody in Bangkok on an unrelated sex charge when he was arrested in the Ramsey case. Later The A.P. quoted Lt. Gen. Suwat Tumrongsiskul, the head of Thailand’s immigration police, as saying he was unaware of any criminal charges against Mr. Karr in Thailand. The A.P. quoted another Thai official as saying Mr. Karr had denied committing a crime at home.

The arrest put a fresh spotlight on a case that once dominated newspaper headlines, television newscasts and supermarket tabloids. For years, as investigators followed hundreds of leads but seemingly made little headway, John and Patsy Ramsey lived under what Ms. Lacy’s predecessor once called “the umbrella of suspicion.”

The Ramseys repeatedly denied any involvement in their daughter’s death, on Christmas night in 1996, even as one detective who had worked on the case speculated that Mrs. Ramsey had struck JonBenet by accident and then wrote a ransom note to deflect attention.