More than two dozen Jewish community centers across the U.S. reported receiving false bomb threats on Wednesday. It's the second wave of bomb threats in two weeks: On Jan. 9, 16 community centers received threats in a single day.

No actual bombs have been found, according to the JCC Association of North America, and many centers have already reopened and resumed regular operations.

The FBI is investigating "possible civil rights violations in connection with threats," The Associated Press reports.

David Posner of the JCC association said in a statement that the group is "concerned about the anti-Semitism behind these threats."

"While the bombs in question are hoaxes," he says, "the calls are not."

The threatening calls led the Anti-Defamation League to issue a security advisory to Jewish institutions across the country.

Bomb threats were made in at least 18 states, the ADL says: New York, New Jersey, Ohio, Florida, Massachusetts, Maryland, Michigan, Minnesota, Delaware, Connecticut, Alabama, California, Maine, Tennessee, South Carolina, Missouri, Texas and Kansas.

Reuters has more on the earlier wave of calls:

"The Jan. 9 threats targeted 16 Jewish community centers in nine U.S. states, prompting the FBI to look into the source of the calls, some of them made using an automated 'robocall' system."It was not immediately clear if there was overlap between centers that received calls on Jan. 9 and those that received them on Wednesday, but the volume of threats was unheard of, said Paul Goldenberg, national director of the Secure Community Network, a nonprofit that advises Jewish groups on security."Copyright 2017 NPR. To see more, visit