Four people have been killed in a car bomb set off by a mobile phone, Mexican authorities say.<br /><br />Police say the attack is the first of its kind by a drug gang against security forces.<br /><br />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.<br /><br />Al Jazeera's Franc Contreras reports from Mexico City. (July 17, 2010)
