Gunmen also killed 20 people in another drug-plagued northern city.
The killings came just weeks after authorities discovered 55 bodies in an abandoned silver mine, which are presumed to be victims of the country's drug violence.
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Hide AdThe bullet-riddled bodies of 18 men and two women were found yesterday in five different parts of Ciudad Madero, a city in the north-eastern state of Tamaulipas, where violence has surged this year amid a turf battle between the Gulf cartel and its former ally, the Zetas gang of hitmen.