A 69-year-old man is being questioned by police, investigating the sending of hundreds of malicious letters to educational establishments across the country, including West Yorkshire.
The suspect, from Liverpool, was arrested yesterday on suspicion of malicious communication and public order offences.
A spokesman for the North East Counter Terrorism Unit said the man had been detained for questioning and inquiries were ongoing.
The arrest followed the release of CCTV images of a man travelling on a train from Liverpool to Ormskirk.
The images were released by West Yorkshire Police, which is leading the investigation into allegations that hundreds of malicious letters were sent predominantly to schools and colleges across the UK. Some were received by educational establishments in West Yorkshire.