An 18-year-old has been arrested after a number of schools and colleges across the UK received threatening emails.
The National Crime Agency (NCA) said the incident was linked to a hoax bomb threat that closed hundreds of schools last week.
The teenager was arrested in Abbots Langley, Hertfordshire, on Wednesday night on suspicion of making malicious communications.
A NCA spokesman said the threats had "caused considerable concern".
He added: "We do not assess there to be an increased risk to the public."
Around 400 schools were closed last week after receiving the emails.
A 19-year-old man was arrested in Watford last week on suspicion of blackmail and making malicious communications after the hoax bomb threat.