Mexican soldiers patrol the beach of Tulum while investigators reconstruct a crime scene in the once-sleepy Caribbean resort, reeling from the deaths of two tourists caught in gang crossfire. “Are they filming a movie?” asks Lukas Smith, a Canadian tourist visiting the town known for its ancient Mayan ruins, turquoise waters and — more recently — dance music parties. However, the troops and forensic experts working at a restaurant in downtown Tulum are not actors. It was there that a shootout between rival drug dealers last month claimed the lives of German tourist Jennifer Henzold, 35...