Four people have been killed in a car bomb set off by a mobile phone, Mexican authorities say.
Police say the attack is the first of its kind by a drug gang against security forces.
Nearly 25,000 people have been killed in drug-related violence since Felipe Calderon, the Mexican president, took power in 2006 and stepped up the fight against drug gangs.
Al Jazeera's Franc Contreras reports from Mexico City.
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