Two investigative state police officers were shot and killed in Tlajomulco after they stopped a tanker (pipa) they believed was carrying stolen Pemex gasoline.
The officers from the Fiscalia General de Jalisco were checking out the tanker and its lone driver when they were ambushed by a group of men who appeared in two vehicles.
The officers were unable to offer any significant resistance, police reports said.
More than 100 bullet shells were found at the crime scene.
Authorities say drug cartels have now commandeered the black market in stolen gasoline, which was for years operated by small-time criminals.