A teenager has been arrested after schools and colleges across the UK received threatening emails.
The National Crime Agency (NCA) said it was believed the incident was linked to a hoax bomb threat that closed hundreds of schools last week.
An 18-year-old man was arrested by Hertfordshire Police and the NCA in Abbots Langley on Wednesday night on suspicion of making malicious communications.
A spokesman for the NCA said: "We understand these threats have caused considerable concern but we do not assess there to be an increased risk to the public."
A 19-year-old man was arrested in Watford last Wednesday on suspicion of blackmail and making malicious communications after the mass bomb hoax.