A disgruntled gamer is thought to be behind a hoax email sent to about 400 schools in England threatening to bomb school grounds if money was not paid.
A number of schools were evacuated but police said there was no evidence the threats were terror-related.
Schools in London, Manchester and North Yorkshire were among those affected.
The email appeared to come from gaming server VeltPvP, but the company said the account had been "spoofed" and the message had not come from them.
The US firm's 17-year-old CEO told the BBC he suspected the hoax emails had been sent by a disgruntled Minecraft player in a bid to damage VeltPvP's reputation.
The email address requested payment to an email address that used the domain veltpvp.com but the website posted messages on Twitter denying any involvement.
It said: "We have nothing to do with the bomb threats that were sent out to the 400+ UK schools.
"We're extremely sorry for anyone who had to deal with this, but just know it's fake."
'Kids' playing Minecraft
VeltPvP.com is a US-based online "player-versus-player" server that allows gamers to compete against other users in the world-building game Minecraft.
Carson Kallen, the 17-year-old CEO of the firm, told the BBC he had a team of 50 people managing 10,000 users a day.
He says: "Everyone who plays it is between the ages of eight and 18 years old - it's all kids playing."
Mr Kallen told the BBC: "Every now and then we have a little rebel who will try to do something bad like this. We've had stuff like this happen before but nothing this extreme."
He said it was likely that a disgruntled user orchestrated the attack: "He was probably a player who got banned from our server and got mad. This is his way of trying to make us look bad."
Sarah Stephenson was at her son's school, Oathall Community College in West Sussex, waiting for a meeting when it was evacuated.
"I noticed a little bit of activity going on. I went through to student support and all of a sudden a notification went out that the school was being evacuated.
"My son George was in class at the time. The students were evacuated out of the school building and told to ring their parents. An email was sent out to all the parents."
Another affected school, Dowdales secondary in Cumbria, tweeted: "As you may know the school office received a threatening email this morning.
"The decision was taken to evacuate the school as a precaution. Students and staff are safe and have returned to normal lessons."
Schools and colleges in Cumbria, Cambridgeshire, East Yorkshire, Hertfordshire, Lincolnshire, West Midlands, Derbyshire, Avon and Somerset and Northumbria were also targeted.
There was a national threat made to lots of schools this morning and KKS went into invacutation to ensure the safety of our site.This was in conjunction with CST and the Metropolitan Police. We have established that there is no further threat and School is now running as normal.— Kantor King Solomon (@kingsolomonhigh) March 19, 2018
Both the Home Office and the Department for Education advised any school that received the email to contact their local police force - even though a version of the email that has been posted on Twitter warns the schools not to.
Humberside Police said 19 schools in its area received the threat. Det Supt Tony Cockerill, from the force, said: "We have spoken to all schools who have contacted us, reassured them that there is no need to evacuate and offered them security advice."
Greater Manchester Police's Assistant Chief Constable Vanessa Jardine said she understood parents and the general public would be very concerned, adding: "I want to assure you that we are working closely with all of the schools to ensure the safety of pupils and staff.
"We are carrying out extensive enquiries to understand the full circumstances and although there is not currently believed to be any direct threat, as with any report of this nature, they are all being thoroughly investigated."
A North Yorkshire Police statement described the threat as a "hoax", adding: "Our cybercrime unit detectives, supported by local officers, have looked at these incidents and it is not believed there is any genuine threat."