Five people have been killed and three were wounded after gunmen opened fire at a social club in the city of Monterrey in northwestern Mexico, local authorities say. It is not exactly known why the club was targeted but authorities believe one of the reasons could have been a failure to pay protection money to gangsters. Monterrey has become a battleground for two criminal gangs – the Gulf Cartel and the Zetas. Last July, over 20 people were killed when a gunman opened fire at another bar in the town.