PORTLAND, Maine — A Jewish preschool and community center in Portland was evacuated Wednesday morning after someone called in a bomb threat — part of a wave of at least 20 similar cases across the country.

The Jewish Community Alliance of Southern Maine on Ashmont Street received a phone call to the front desk at around 9:30 a.m. from a person who said a bomb would be detonated in the building, according to Executive Director Ellie Miller.

The call included anti-semitic statements, according to a statement from the Portland Police Department. Police are investigating the case as a hate crime and referred it to the Civil Rights Unit of the Maine attorney general’s office, said spokesman Lt. Robert Martin.

The roughly 40 students, ages 3 to 5, and 18 staff members exited the building within a few minutes of the threat, which was immediately reported to the police, Miller said.

Classes resumed just over an hour later, after police had cleared the building with a bomb-sniffing dog, Miller said.

“As far as the kids know, they went on a walk in the snow,” Miller said. “They’re little, and they’re fine.”

The call was among at least 20 threats made against Jewish organizations across the U.S. on Wednesday. Threats also were made against multiple American Jewish groups on two dates earlier in January. It wasn’t immediately clear whether Wednesday’s threat in Portland was part of an organized national effort. Federal authorities are working with state and local law enforcement in Maine and nationally to investigate the issue, according to an FBI spokeswoman.

“The FBI and the Justice Department’s Civil Rights Division are investigating possible civil rights violations in connection with threats to Jewish community centers across the country,” Kristen Setera said in a statement.

Online, the parents of students expressed anguish and confusion over the incident.

“I can’t fathom what side of history you are on where this is OK in any way, shape or form,” Briana Volk wrote on Facebook.

Rabbi Jared Saks of South Portland’s Congregation Bet Ha’am pointed to the trend of threats against Jewish groups across America and the recent vandalism of a halal grocery store in Portland as evidence that hateful people have been emboldened by a political climate that has brought white nationalism into the mainstream. After someone smashed the windows of the Ahram Halal Market on Christmas Eve, Portlanders turned out in large numbers to support local Muslim businesses.

“This isn’t only a threat against the Jewish community, it’s a threat against the broader Portland community and a threat against the American community as well,” said Saks. “It is a result of the political climate, but the response makes it clear that it’s something that will make us stronger as a nation.”