Well, it's not really luck. It's exactly because there's cctv in enough places that they were able to get his photo from the one moment he lowered his mask at the hostel, but even more significantly that they were able to piece together his movements enough to be able to suspect that the guy at the hostel was the same one that carried out the shooting. Then it was a matter of distributing the photo and hope someone recognizes him. And offering the reward ofc. Very simple but not luck.