Gunmen engaged state police in a running shootout that killed five officers and possibly one of the attackers in the western state of Jalisco, authorities said.
The state attorney general's office said a convoy of seven vehicles carrying at least 30 gunmen approached state policemen on a local highway and opened fire.
Four policemen died in the initial fighting and another officer was killed when police gave chase. A body was found in one of the vehicles involved and might be that of an assailant.
The attackers had not yet been identified.
More than 13,500 people have died in drug-related violence since President Felipe Calderon launched an offensive against cartels in late 2006.