According to todays ‘Corriere della Sera’ the offender was initially suspected when security found a strange items in his possession at the security check for a flight between Manchester (UK) and Orio al Serio (Milano-Bergamo, Italy) , but they could not do anything as the item was found not to be a bomb/explosive.
However, UK security iformed Italian police. Apparently, the offender lived in Italy (or has lived in Italy for some years) and was working in UK.
Upon arrival in Italy, Italian police followed him discretely in all his movement, and they found that he had contacts with activists.
Upon his return in UK, before boarding the a/c in Italy, he was thoroughly inspected without finding any dangerous item. During his Italian trip, however, UK police determined that the item that they previously impounded from him in Manchester were in fact part of a homemade bomb, and therefore upon aiirval in Manchester- and in cooperation with the Italian police they arrested him