A teenager has been arrested after a mass bomb hoax closed hundreds of schools across the UK.
The National Crime Agency (NCA) said there was "no credible threat to the emails" received this week.
A 19-year-old man was arrested by Hertfordshire police in Watford on Wednesday evening on suspicion of blackmail and making malicious communications.
The NCA said in a statement: "We understand parents' and teachers' concerns but stress there was no credible threat to the emails schools received.
"However we are taking the communications extremely seriously.
"As this is a live investigation we are unable to comment further."