Jewish centers in York and Harrisburg were among those affected by a wave of bomb threats in at least 11 states Monday. This came the day after more than 100 headstones were knocked over at a Jewish cemetery in Philadelphia.

No bombs were found. Gov. Tom Wolf condemned the acts.

“These acts are cowardly and disturbing,” said Wolf. “We must find those responsible and hold them accountable for these hate crimes. As always, the safety of Pennsylvanians is my top priority, and I do not take these threats and acts lightly.”

Between 400 and 500 students, staff and volunteers were evacuated from the York Jewish Community Center after a bomb threat was called in at around 10 a.m. No bombs were found there or at any of the targeted facilities and operations were back to normal by mid-afternoon.

The JCC’s emergency protocol and evacuation plan was used for the first time. Melissa Plotkin, director of community engagement and diversity, said it went smoothly.

“The incidents that have been happening across the United States, there is general concern, which I think anyone rightfully would have,” Plotkin said. “Our children handled it very well, our staff handled it wonderfully.”

Wolf said he’s directed the state police and state Department of Homeland Security to look into all three incidents. The FBI and the U.S. Justice Department's Civil Rights Division are investigating the threats. This was the fifth wave of bomb threats against Jewish community centers and Jewish institutions since January.

Wolf said he considers them all acts of terrorism.

The Associated Press contributed to this report.