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: