A 19-year-old man has been arrested after a mass bomb hoax closed hundreds of schools across England.
The teenager was arrested in Watford, Hertfordshire, on Wednesday night, on suspicion of blackmail and making malicious communications via email.
The National Crime Agency (NCA), which is leading the investigation, said there was "no credible threat" from the emails received by schools.
However, it added it was "taking the communications extremely seriously".
Around 400 schools were closed on Monday after receiving the emails.
Schools in London, Manchester and North Yorkshire were among those affected.