Authorities say bomb threats sent to dozens of schools, government buildings and other locations across the United States appear to be a hoax.
Law enforcement agencies across the country dismissed Thursday's threats, which they said were meant to cause disruption and compel recipients into sending money and are not considered credible.
Some of the emails had the subject line: "Think Twice". The sender claimed to have had an associate plant a small bomb in the recipient's building and that the only way to stop him from setting it off was by making an online payment of $US20,000 ($A$27,600) in Bitcoin.
"We are currently monitoring multiple bomb threats that have been sent electronically to various locations throughout the city," the New York City Police Department's counterterrorism unit tweeted. "These threats are also being reported to other locations nationwide & are NOT considered credible at this time."
Other law enforcement agencies also dismissed the threats, which were written in a choppy style reminiscent of the Nigerian prince email scam.
The Palm Beach County, Florida, sheriff's office and the Boise, Idaho, police said they had no reason to believe that threats made to locations in those areas were credible.
Across the country, some schools closed early and others were evacuated or placed on lockdown because of the hoax. Authorities said a threat emailed to a school in Troy, Missouri, was sent from Russia.
The bomb threats also prompted evacuations at city hall in Aurora, Illinois, the offices of the News & Observer in Raleigh, North Carolina, a suburban Atlanta courthouse and businesses in Detroit.
Penn State University notified students via a text alert about threats to a half-dozen buildings and an airport on its main campus in State College, Pennsylvania. In an update, the school said the threat appeared to be part of a "national hoax".
