Soldiers found 13 bodies inside a vehicle on a highway near Tampico, a port city in the northeastern Mexican state of Tamaulipas, state prosecutors said.

Army troops received a tip while on patrol that a 3.5-ton truck with tags from neighboring Veracruz state had been abandoned on the highway, the Tamaulipas Attorney General’s Office said.

The vehicle was found Sunday on the El Moralillo-Tampico highway, the AG’s office said.

“A pair of banners alluding to gang rivalries” were found with the bodies, the AG’s office said.