New York, NY – A massive wave of bomb threats crossed the United States and Canada from coast to coast on Thursday.

Investigators said that threatening emails – some bearing the title “Think Twice” – showed up in inboxes in most major North American cities, the New York Post reported.

Some of the bomb threats were sent to schools – including the infamous Columbine High School that was the scene of the first major U.S. school shooting – causing campuses to go into lockdown and to increase security.

Penn State was one of the universities that got the threat, and school officials responded with an increased police presence on campus, according to the New York Post.

Different bomb threats were sent to different entities and it’s not known which messages were received by the schools.

The New York Police Department announced that the bomb threats were “not credible” and were an attempt to extort bitcoin, a popular cryptocurrency, from the recipients.

“Hello. There is an explosive device (Tetryl) in the building where your company is conducted,” read the opening of one letter, sent from a spoofed email address to hundreds of businesses, schools, hospitals, and other buildings on Dec. 13.

“It is small and it is hidden very carefully, it cannot damage the structure of the building, but in case of its explosion you will get many wounded people,” the letter continued.

The letters went on to say that they were not part of a terrorist organization, but that the buildings were being watched by a “recruited person.” They also threatened to detonate the bombs if the recipient of the email called the authorities.

“I want to offer you a bargain. 20.000 dollars is the cost for your life. Pay it to me in BTC [Bitcoin] and I warrant that I have to withdraw my mercenary and the device won’t explode,” the threat read.

The sender provided an online address for sending the ransomed bitcoin and demanded payment before the end of business on Thursday.

“If the workday is over and people start leaving the building the bomb will detonate,” the letter continued. “This is just a business, if you don’t send me the bitcoin and the bomb detonates, next time other companies will pay me more money, because it isnt [sic] a one-time action.”

Despite the blanket announcement that the threats were not credible, police and fire services in many major cities spent the day running calls for the fake bomb threats.

A spokesperson for the Suffolk County police on Long Island said their department “responded to at least 11 bomb threat incidents during which businesses, one school and one medical facility received an email demanding money.”

The Federal Bureau of Investigation (FBI) was participating in the investigations nationwide, but noted that a similar widespread threat was distributed a year ago.

“We are aware of the recent bomb threats made in cities around the country, and we remain in touch with our law enforcement partners to provide assistance,” the FBI said in a statement. “As always, we encourage the public to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety.”

Bitcoin values dropped significantly in the wake of the threats, according to Business Insider.

On Thursday afternoon, Bitcoin was down six percent to almost $3,300 per coin.