After assembling suicide bomb vests for the attacks that slaughtered 130 people in Paris last November, Najim Laachroui went underground in his native Brussels.

The 24-year-old explosives expert wasn’t just hiding from the biggest manhunt in Europe’s recent history. He was plotting. In a dingy apartment converted into a bomb factory, Laachroui exchanged a series of messages in French with Abu Ahmed, a shadowy commander in the Islamic State based in Syria.

If law enforcement agencies had intercepted the communications, they would have been immediately alarmed. Laachroui asked militants in Syria to test chemical mixtures so he could assemble powerful bombs. He discussed his hopes to strike France again and disrupt a soccer championship there. He reported that he and half a dozen other fugitives from the Paris attacks had split up among three safe houses, according to Belgian and French counterterror officials.