MONTERREY, Mexico (AP) — This wasn't supposed to happen in Monterrey. Mexico's modern northern city's image had been one of gleaming glass towers set against the Sierra Madre with world-class universities, including Mexico's equivalent of MIT.Then, gunmen chase a security guard into a crowded downtown shopping promenade and open fire, leaving a 21-year-old university student lying dead from a gunshot to the head.Drug violence has hit Monterrey seemingly overnight asthe Gulf and Zetas gangs fight for control of Mexico's third-largest — and wealthiest — city.It happened nearly overnight, baring issues that plague Mexico: a lack of credible policing and the habit of ignoring the drug trade as long as it's orderly and peaceful.