Fourteen decapitated corpses were found elsewhere in the city, the Los Angeles Times reported. The bodies were in trash bags left in a truck outside a government building, while the heads were in three ice chests discovered near city hall.
Nuevo Laredo in the state of Tamaulipas lies across the Rio Grande from Laredo, Texas, making it an important drug shipment center. Much of the recent violence appears to come from an attempt by the Sinaloa drug cartel to move into territory that has been controlled by the Zetas, investigators say.
The two groups are the largest cartels.
The banner at the overpass warned against "heating up the plaza," presumably engaging in violence that would prompt a federal crackdown.
Most of the victims were men. Four women were among those at the overpass.
Investigators said all appeared to be in their late 20s.