A grenade exploded outside police headquarters in Monterrey, Mexico on Saturday injuring two policemen and causing damage to the headquarters building.
Army officials and police are investigating the incident as authorities have seen similar incidents in the past three months at judicial buildings, media offices, and nightclubs.
The state of Nuevo Leon has seen a wave of intense violence in the struggle between the two major drug cartels - the Gulf and Los Zetas.
The violence has claimed around 500 lives in the city since March 2010.
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