A wave of bomb threats emailed to hundreds of schools, businesses and government buildings across the US triggered searches, evacuations and fear - but there were no signs of explosives, and authorities said the scare appeared to be a crude extortion attempt with hoaxers requesting cryptocurrency ransoms.

Law enforcement agencies across the country dismissed the threats, saying they were meant to cause disruption and compel recipients into sending money and were not considered credible.

Some of the emails had the subject line: "Think Twice." They were sent from a spoofed email address. 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 $20,000 (£15,800) 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.