The Jalisco state attorney general's office confirmed 11 heads and other body parts were found in a Toyota Sienna, and seven heads and body parts were found in a Ford Ecosport, El Informado reported.
The Guadalajara Reporter said the grisly discovery was made just off the Guadalajara-Chapala highway in the municipality of Ixtlahuacan de los Membrillos.
Jalisco Attorney General Tomas Coronado Olmos said the killings were in retaliation for the recent slayings of 23 people in Nuevo Laredo in Tamaulipas state in northeastern Mexico, the Reporter said. Nine of those victims were left hanging from a road bridge.
Mexico has been wracked by drug cartel violence for years, with decapitations a signature method often used by gangs.