Bomb threats sent to hundreds of schools, universities and government buildings across the US all appear to have been a hoax, authorities have said.
Beginning on Thursday morning, emails arrived in the inboxes of staff at city halls, courts and businesses in a number of states, each claiming that a bomb had been planted at their building.
The sender issued the recipients with an ultimatum, saying that the only way to prevent the explosive from being detonated was to make an online payment of $20,000 (£15,810) in Bitcoin.
Law enforcement agencies have dismissed the threats, but not before they forced nationwide evacuations.
Among the buildings to have been cleared out or placed on lockdown were Aurora City Hall in Illinois; a suburban court in Atlanta; and the offices of the News and Observer in Raleigh, North Carolina.
Businesses in Detroit, New Orleans and Alaska were also targeted, as was Penn State University in Pennsylvania.
The university later told students via a text message that the threat appeared to be part of a "national hoax".
In a tweet, the New York City Police Department – among the agencies to have responded to similar threats – confirmed that the emails were "not considered credible".
"We are currently monitoring multiple bomb threats that have been sent electronically to various locations throughout the city," its counter-terrorism unit tweeted.
"These threats are also being reported to other locations nationwide and are not considered credible at this time."
Police in Florida and Idaho echoed that sentiment, although the FBI urged people to "remain vigilant" while forces across the country investigated where the emails may have come from.
Authorities in Missouri said a threat emailed to a school in the city of Troy was sent from Russia.
The flurry of emails – many of which were sent with the subject line "think twice" – are not thought to be connected to a bomb threat made by phone to Columbine High School on Thursday morning.
Police said officers were looking for the person responsible for making the call, with the individual having claimed to have placed explosive devices in the school and to be hiding outside with a gun.
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Jefferson County Sheriff's Office spokesman Mike Taplin said no explosives had been found at the school, where 12 students and a teacher were killed during one of the worst mass shootings in US history in 1999.
The threat on Thursday drew a large police response and students at 23 other schools nearby were also temporarily placed on lockdown, but classes continued normally.