I think the concern was probably less about foiling a terrorist attack as worry that he was a cross person who might take extreme action.<p>The questions about his life to me indicate that was their main worry. You know; of he just lost his job or something and life was spiralling out of control etc etc.<p>In a way I can see why they investigated. Arresting him was too much clearly  but you can't just ignore an explicit threat entirely - ESP if it is reported.