WASHINGTON (Reuters) – A rash of bomb threats were emailed on Thursday to hundreds of businesses, public offices and schools across the United States and Canada demanding payment in cryptocurrency, but none of the threats appeared credible, law enforcement officials said.

Initial jitters sparked by the wave of awkwardly worded messages, threatening to set off explosives unless $20,000 were delivered in bitcoin, subsided within hours as some local authorities began referring to the electronic extortion notes as a scam.

One email targeting a St. Louis-area middle school was traced by local investigators to an internet protocol, or IP, address in Moscow, the sheriff’s office in Lincoln County, Missouri said.

But U.S. government sources speaking on background to Reuters said that such findings were inconclusive and that federal investigators doubted that Russians or the Russian government were involved.

The officials cautioned that such an IP address does not prove it came from Russia because the sender could have electronically laid a false trail to cover up its true origin. They suggested instead that the flurry of emails were part of a wide-scale digital hoax.

The security scare began shortly before 2 p.m. EST (1900 GMT) as police departments in major U.S. cities coast to coast began reporting on Twitter that numerous local businesses had received the menacing emails.

Six hours into the security scare, no actual explosives had been found, authorities said. But the threats prompted brief evacuations of a Toronto subway station and a newspaper office in Raleigh, North Carolina. Some public schools and businesses also were evacuated as a precaution.

Lincoln County, Missouri, sheriff’s spokesman Lieutenant Andy Binder said authorities bused the students from a middle school receiving one of the threats to another campus as a precaution, but the school was later determined to be safe and classes will resume there on Friday.

Among other cities where bomb threats were reported by authorities were Washington, New York, Detroit, San Francisco, Los Angeles, Phoenix, Oklahoma City, Grand Rapids, Iowa, Denver, Ottawa, and Calgary, Alberta.

Several hours after North America was hit, similar email threats were showing up in New Zealand, according to police there.

‘SCAM HAPPENING NATIONWIDE’

Police at the University of Wisconsin in Madison tweeted an image taken of one email threat found to be circulating that said in part: “Good day. There is an explosive device (lead azide) in the building where your company is conducted. It is assembled according to my guide. It is compact and it is covered up very carefully. It can not damage the structure of the building, but in case of its explosion you will get many wounded people.”

The campus police tweet concludes by referring to the emails as “a SCAM happening nationwide.”

The FBI and other federal agencies were alerted to the email chains, officials said.

“We are aware of threats being made in cities across the country,” Rukelt Dalberis, an FBI spokesman in Los Angeles, told Reuters separately. “We remain in touch with our law enforcement partners. We encourage the public to remain vigilant and report suspicious activities that could represent a threat.”

A similar wave of emailed hoax bomb threats in December 2015 prompted officials in Los Angeles to close the city’s public school system, which national law enforcement officials later criticized as an overreaction.

Two weeks previously, a married couple inspired by Islamic State had killed 14 people at a California county office building in a shooting rampage.

A teenager with dual Israeli-U.S. citizenship was arrested in Israel in March 2017 for making bomb threats to more than 100 Jewish organizations and Jewish community centers in dozens of U.S. states over several months.