MEXICO CITY (Reuters) - Gunmen burst into a bar in the Mexican city of Monterrey and shot dead at least 20 people Friday, the latest massacre in the city battered by drug-gang violence, El Universal newspaper reported. The gunmen fired AK-47 assault rifles throughout the bar in downtown Monterrey, Milenio newspaper reported. Several other Mexican media outlets, including Reforma newspaper, reported the same tally of dead and injured. Over the last four years, industrial and relatively prosperous Monterrey has gone from being a model for developing economies to a symbol of Mexico's drug war chaos.