The recent spate of anti-Jewish incidents—from the over 100 bomb threats to JCCs around the U.S. (and Canada), as well as two such calls targeting ADL locations in New York and San Francisco—is undoubtedly alarming. Frustratingly, clear information about the origin and motive(s) behind these calls is hard to come by.

These bombs threats (timeline here), along with two high-profile instances of vandalism to Jewish cemeteries in St. Louis and Philadelphia, both came on the heels of months of anti-Semitic incidents on social media that crescendoed during the 2016 presidential campaign. Unlike the phone calls, an instance of someone shooting a bullet at a classroom window at an Indiana synagogue, which happened on Monday, and the cemetery desecrations, are concrete acts of violence.

As of this publishing, more than 100 JCCs have been targeted by bomb threats, from Davie, Florida, and Long Island, New York, to Owings Mills, Maryland and Las Vegas, Nevada. On Feb. 28 alone, 23 such threats were targeted at JCCs. In Birmingham, Alabama, the JCC has been evacuated three times; its director called the threats “very difficult, very challenging, very fearful.”

No bombs have been found at any of the JCCs.

Reports explain that the calls are either “robo-calls” or made live—both of which use “voice disguising technology.” In Milwaukee, Wisconsin, a switchboard operator from a call center entered into a question-and-answer conversation with one caller.

Here is what one bomb threat, targeted at a JCC in Wilmington, Delaware, sounded like.

Phone numbers have been thus far undetectable and a theory that the calls are coming from a single source has been floated by a security professional who works with the FBI and the U.S. Department of Homeland Security. CNN is reporting that officials believe many of the calls are coming from overseas. The New York Times says that the FBI began to look into these threats in January; that they believe a lone individual may be behind the calls; and that that individual may be using an “internet calling service.”

The FBI has offered little in the way of clarity, although the organization has said the threats are a priority. Stephen Richardson, the bureau’s assistant director for the criminal investigative division, told The New York Times: “Agents and analysts across the country are working to identify and stop those responsible. We will work to make sure that people of all races and religions feel safe in their communities and in their places of worship.”