CNN  — 

Authorities across four countries are trying to learn who sent dozens of email bomb threats Thursday afternoon, causing anxiety and business disruptions but no reported violence.

Guy Dickinson tweeted this photo of  police at 17th Street and K Street Northwest in Washington, DC on Thursday afternoon.

At this time it’s unclear if the threats – which have been received in San Francisco, New York, Seattle, Miami, Washington, DC and other locations nationwide – are connected. The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives said they are aware of the threats and are working with law enforcement to provide assistance.

“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.

Email threats also have been received in Vancouver, Ottawa and Toronto, Canada.

Vancouver Police Department Sgt. Jason Robillard tells CNN that businesses have received threats. He is not aware of any buildings that have evacuated.

An email demanding $20,000 via bitcoin was forwarded Thursday to CNN affiliate KOCO-TV in Oklahoma City by a viewer who received it at her business.

The message was identical to an email warning posted on social media by the police department in Cedar Rapids, Iowa, and it was similar to descriptions of other threats posted on social media nationwide.

The message was identical to an email warning posted on social media by the Cedar Rapids, Iowa Police Department and similar to descriptions of other threats posted on social media nationwide.

The Cedar Rapids Police Department “has found NO CREDIBLE EVIDENCE that these emails are authentic. It appears to be a robo-email that has been sent throughout the area hoping to scam businesses out of money. We have also received information that businesses in surrounding counties may have also received this email,” the agency posted.

CNN is not disclosing the name of the sender or specifics of the bitcoin account.

Similar threats were sent earlier

It is unclear if Thursday’s threats could be related to a series of hoax bomb threats that have been sent via an online messaging service to recipients in multiple states since late August.

CNN has learned that over more than three months, academic institutions, federal and state law enforcement agencies, gun stores, and municipal offices in more than a dozen states received threatening emails deemed hoaxes by law enforcement authorities.

In a warning reviewed by CNN, the FBI recently shared information with law enforcement authorities nationally about the prior emails sent by an email service called “Guerilla Mail.”

The bulletin indicated the FBI has not identified any real or hoax devices at the threatened locations, suggesting law enforcement deemed at least some of the threats credible enough to conduct searches.

Threats at universities, media, courthouse

In Seattle, the University of Washington noted in a campuswide alert that the FBI had “advised that the email is not a credible threat.”

The Thurston County Courthouse in Olympia, Washington, and the Park Record newspaper in Park City, Utah, also received similarly threatening emails. People were evacuated but eventually were allowed back inside, according to tweets on verified Twitter accounts.

In California, the Riverside Sheriff’s Office had “an influx of email threats” and is taking them seriously, although no threat has been substantiated.

The San Francisco Police Department responded to reports of bomb threats at locations throughout the city.

“We have received information that several other cities across the United States have received similar threats,” police said.

Pennsylvania State Police are “investigating some bomb threats in the eastern part of the state,” a department spokesman fold CNN.

University police later said the threat appears to be a hoax.

The Chicago Police Department by Thursday afternoon was investigating threats at 36 locations, with “no reports of any injured people, any suspicious objects or any evacuations,” police spokesman Anthony Guglielmi tweeted.

“#ChicagoPolice are working with federal partners on the investigation, and at this time there is no elevated threat level for the city of Chicago,” he tweeted.

Threats were also emailed to the Charlotte News & Observer and the Raleigh News & Observer newspapers in North Carolina.

Dozens more threats continued to come in across the country later into the day, authorities reported.

CNN’s Jeremy Grisham and Hollie Silverman contributed to this report.