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. (July 17, 2010)