While authorities investigate what set off a deadly shooting at an electronic music festival event, residents said it came amid a growing and increasingly open drug scene in this resort town that has long been spared the violence of Mexico’s cartel wars. Concerns that violence may be creeping into Playa del Carmen were voiced as people attended a Monday evening vigil in front of the Blue Parrot nightclub, where five people died and 15 were wounded or injured in the chaos before dawn.