WASHINGTON, Dec. 13 (Xinhua) -- Multiple institutions across the United States on Thursday received emails and phone calls threatening bomb attacks, which caused evacuations and law enforcement actions.

The Federal Bureau of Investigation (FBI) said in a statement it was aware of the recent bomb threats made in cities around the country. "We remain in touch with our law enforcement partners 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.

The New York Police Department tweeted that it was monitoring "multiple bomb threats," and "these threats are also being reported to other locations nationwide & are NOT considered credible at this time."

The CNN reported that it was not yet clear whether these threats, which had been received in San Francisco, the University of Washington in Seattle, the Pennsylvania State University in Pennsylvania, were connected.

"We have received information that several other cities across the United States have received similar threats," police from San Francisco Police Department said.

The University of Washington in Seattle issued a campus-wide alert saying the FBI had "advised that the email is not a credible threat," and concluded sweeps of possibly affected buildings.

The Pennsylvania State University said in a statement its police was coordinating with the FBI in "investigating a message received by individuals in multiple locations on campus and across the state," adding that the threat "does not appear to be a legitimate threat, however, an investigation is ongoing."

The police later said that the threat "appears to be a hoax."

Earlier in the day, a false bomb threat was made against Columbine High School in Colorado, causing the school and 23 other schools nearby to be placed on lockdown.

More threats were still being reported across the country.