U.S. officials said bomb threats were called into 13 Jewish community centers and seven Jewish day schools in a dozen states across the country.

No bombs were found Monday, and the JCC Association of North America said normal operations have resumed at all 20 buildings.

The group said it was the fifth wave of bomb threats at Jewish institutions around the country since January. The group counted a total of 89 incidents in 30 states and Canada.

In Philadelphia, meanwhile, police are investigating after hundreds of headstones were knocked over at a Jewish cemetery during the weekend.

Attorney General Jeff Sessions told reporters on Monday that bomb threats made against Jewish groups are "unacceptable" and a "very serious and destructive practice."

Pennsylvania Gov. Tom Wolf called the bomb threats and cemetery vandalism reprehensible.