(CNN) Dozens of businesses and institutions across the United States and Canada received email threats Thursday afternoon, prompting evacuations and sweeps of buildings.

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.

This email demanding $20,000 via Bitcoin was forwarded to CNN affiliate KOCO-TV in Oklahoma City by a viewer who received it at her business. It's unclear whether everyone who received a threat on Thursday received the same email.

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 posted: "The 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."

CNN is not disclosing the names of the sender and recipient 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 on Thursday, the University of Washington searched possibly affected buildings after receiving threats. It then noted in a campus-wide 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 have been allowed back inside buildings at those locations, according to tweets on their 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 spokesman for the department told CNN.

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

The Chicago Police Department received 15 to 20 reports of emailed threats in the past few hours, according to Officer Jennifer Bryk.

Director of Communications 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."

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.