A wave of bomb threats emailed Thursday to hundreds of schools, businesses and government buildings across the US triggered searches, evacuations and fear — but there were no signs of explosives, and authorities said the scare appeared to be a crude extortion attempt.

Some 500 people were safely evacuated from the Jewish Community Center of San Francisco after a nearby bank was targeted in the hoax.

San Francisco Police were called to at least nine locations around 10 a.m., including the JCC, which was ordered to evacuate after a threat was made against the San Francisco Fireman’s Credit Union, located directly across the street.

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Just before 11 a.m., JCC executive director Marci Glazer received a call from the police, informing her of the bomb threat and the need to get people out of the building.

“This is something we are completely prepared for and practice on a regular basis,” she said.

JCC staff accounted for preschoolers, gym users, swimmers in the pool, seniors and others, and ushered them out of the building within eight minutes, according to Glazer. The all clear notice came less than two hours later, when people were allowed back in the building.

By 1 p.m., the JCC’s website announced that the building and the fitness center had reopened. The preschool remained closed for the day, and afterschool has been canceled.

Police presence seen outside the @JCCSF. SFPD are investigating reports of bomb threats throughout the city. https://t.co/a8QcA3ui30 pic.twitter.com/ykALXo6ljx — NBC Bay Area (@nbcbayarea) December 13, 2018

Law enforcement agencies across the country dismissed the threats, saying they were meant to cause disruption and compel recipients into sending money and were not considered credible.

Some of the emails had the subject line: “Think Twice.” They were sent from a spoofed email address. The sender claimed to have had an associate plant a small bomb in the recipient’s building and that the only way to stop him from setting it off was by making an online payment of $20,000 in Bitcoin.

“We are currently monitoring multiple bomb threats that have been sent electronically to various locations throughout the city,” the New York City Police Department’s counterterrorism unit tweeted. “These threats are also being reported to other locations nationwide & are NOT considered credible at this time.”

Other law enforcement agencies also dismissed the threats, which were written in a choppy style reminiscent of the Nigerian prince email scam.

The Palm Beach County, Florida, sheriff’s office and the Boise, Idaho, police said they had no reason to believe that threats made to locations in those areas were credible. One of the emails wound up in a spam filter, Boise Police Chief William Bones said.

The FBI said it is assisting law enforcement agencies that are dealing with the threats.

“As always, we encourage the public to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety,” the FBI said in a statement.

Across the country, some schools closed early and others were evacuated or placed on lockdown because of the hoax. Authorities said a threat emailed to a school in Troy, Missouri, about 55 miles (88 kilometers) northeast of St. Louis, was sent from Russia.

The bomb threats also prompted evacuations at city hall in Aurora, Illinois, the offices of the News & Observer in Raleigh, North Carolina, a suburban Atlanta courthouse and businesses in Detroit.

“Organizations nationwide, both public and private, have reported receiving emailed bomb threats today,” Michigan State Police spokeswoman Shannon Banner said. “They are not targeted toward any one specific sector.”

Penn State University notified students via a text alert about threats to a half-dozen buildings and an airport on its main campus in State College, Pennsylvania. In an update, the school said the threat appeared to be part of a “national hoax.”

Officials at Columbine High School in Colorado were dealing Thursday with a bomb threat of a different sort. Students were being kept inside for the rest of the school day after someone called in a bomb threat against the school.

The Jefferson County, Colorado, Sheriff’s Office said the caller claimed to have placed explosive devices in the school and to be hiding outside with a gun.

There is nothing to validate the threat that was found at Columbine, where 12 students and a teacher were killed by two students in 1999, according to Sheriff’s spokesman Mike Taplin.

Two dozen other Colorado schools were also temporarily placed on lockout, meaning their doors were locked but classes continued normally, as the threat was investigated.

Last month the Tel Aviv District Court sentenced an American-Israeli man, convicted of hoaxing US Jewish community centers and other targets around the world with thousands of bomb threats, to 10 years in prison.

The 20-year-old, M., whose full name is withheld by a gag order in Israel, has been diagnosed with autism and also suffers from a brain tumor, which his parents and attorney argue impacted his behavior. But, while Judge Tzvi Gurfinkel acknowledged M.’s medical condition, he concluded that the young man was responsible for his actions and understood the difference between right and wrong.

Authorities say he made thousands of threatening calls, mostly to community centers and schools in the US, from 2015 to 2017, using an online calling service that disguised his voice and allowed him to hide his identity. He also targeted hundreds of airlines and airports, malls, and police stations, in the US, Canada, the UK, New Zealand, Australia and Britain, and tried to extort Republican State Senator Ernesto Lopez from Delaware.