The Federal Bureau of Investigation has been leading an inquiry since January, and a federal law enforcement official, who was not authorized to discuss a continuing investigation, said that a single person may be making the threats using an internet calling service. Independent analysts, including extremism researchers and retired law enforcement officials, share that theory and said that, so far, they have seen no evidence of an organized effort.

Though some people had suspected that the calls were recorded and automated, there was evidence to the contrary. In Milwaukee, for instance, a switchboard operator asked questions and received responses from the caller, said Mark Shapiro, the president of the Harry and Rose Samson Family Jewish Community Center.

Mr. Posner said an F.B.I. official had emphasized that the investigation was a priority for the bureau, involving experts in behavioral analysis, civil rights and hate groups.

“Agents and analysts across the country are working to identify and stop those responsible,” Stephen Richardson, the bureau’s assistant director for the criminal investigative division, said. “We will work to make sure that people of all races and religions feel safe in their communities and in their places of worship.”

According to Mr. Posner’s group, more than 80 community centers and day schools in the United States and Canada have been threatened, some repeatedly. The calls have come in five rounds, most recently on Monday, when there were 31 threats.

Many of the calls have prompted evacuations and bomb sweeps, forcing schoolchildren from classrooms and employees to push cribs full of infants into parking lots. Retirees have been rushed from swimming pools, and offices and streets shut down.

The threats are frequent and alarming, community center leaders said.

“My initial reaction was, ‘This is our turn,’” said Karen Kolodny, the executive director of the JCC of Mid-Westchester in Scarsdale, N.Y., where officials responded to a bomb threat on Monday. “My reaction was not complete shock. We thought it was going to happen at some point.”