KANO, Nigeria—An accountant of the Alren Construction Co. stood nervously on a bloodstained street days after Islamic militants killed at least 186 people here in a wave of car-bombings and armed attacks.

The company recently bought a house next to a police barracks, presuming it safe for expatriate staffers. But on Jan. 20, the day of the citywide attack, teenage militants rammed an explosives-laden car through the barrack gates. Gunmen commandeered the company's guard post, killing three people. At the nearby Farm Centre...