Monterrey and the state it is situated in - Nuevo Leon - have become some of Mexico's most violent places after a turf war broke out between two major drug cartels. More than 600 people have been killed in the last year, and as a result the state and authorities have launched an unprecedented war against drug gangs. As Al Jazeera's Mariana Sanchez reports from Monterrey, fear and frustration have gripped the city.