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 said in a statement its staff were aware of the recent bomb threats made in cities around the country. "We remain in touch with our law enforcement partners to provide assistance."

The New York Police Department tweeted that it was monitoring "multiple bomb threats," and that "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 at locations including San Francisco, the University of Washington in Seattle, Penn State University in Pennsylvania, were connected.