Authorities were investigating on Thursday after a spate of bomb threats prompted evacuations in several cities across the country.

Federal and local law enforcement agencies said they were aware of similar bomb threats made in multiple cities, including Chicago, San Francisco and Washington, D.C. Some officials said the threats were not credible and were believed to be part of an extortion scheme, but urged the public to remain vigilant. At least some were sent by email, officials said.

"The FBI is 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 bureau’s Boston division said in a statement.

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“As always, we encourage the public to remain vigilant and to promptly report suspicious activities which would represent a threat to public safety," the bureau added.

The San Francisco Police Department tweeted that it responded around 10 a.m. local time to reported bomb threats at "numerous locations throughout the city."

"We have received information that several other cities across the United States have received similar threats," the department said. "We are working closely with partner agencies in our city to safely resolve these incidents."

A spokesman for the Chicago Police Department tweeted that officers were responding to similar bomb threats, but that there was no elevated threat level for the city.

The Chicago Tribune reported that officers said the threats were emailed, and included a demand for payment. The threats were believed to be a hoax, officials told the news outlet.

In Washington, D.C., threats were reported in the 1600 block of K Street NW, the 200 block of Pennsylvania Avenue SE and three other locations. Law enforcement blocked off the streets surrounding those areas as they investigated.

Police in nearby Arlington, Va., told NBC Washington that the threats were not credible, but did not elaborate.

Government offices, media outlets and other businesses in New York, North Carolina, Pennsylvania and Florida also reported receiving bomb threats. Individuals at many locations evacuated the buildings as law enforcement arrived, according to multiple reports.