Momodou Keita, a burly, middle-aged man with a mission, sketched out the day’s route in his notebook. Then he grabbed his backpack containing the flier with headshots of 30 women — all suspects — and hopped on his motorbike.

This would be a full day of detective work, and his first stop was the home of Oumu Bah, whose faded photo showed an elderly woman with her mouth set in a grim line. He’d made sure she didn’t know he was coming. The key to the job, he said, is surprise.