The bodies were discovered in a Ford pick-up truck and a Ford Explorer.
The grisly discovery was made around 11 a.m. Sunday, the Tamaulipas state Attorney General's Office said in a brief statement.
The unidentified victims ranged in age from 30 to 35, with one appearing to be older than 50, the AG's office said.
The Gulf drug cartel has been battling Los Zetas, a band of army special forces deserters turned hired guns and drug traffickers, since 2010 in northeastern Mexico, with Nuevo Laredo, Ciudad Mier, Ciudad Miguel Aleman and Camargo caught in the middle of the turf war along the US-Mexico border.
The cartel war has apparently intensified in recent months because of the presence of gunmen from the Sinaloa cartel, which is led by Joaquin "El Chapo" (Shorty) Guzman.