NUEVO LAREDO (AFP) - Troops made a grisly Christmas discovery in northeastern Mexico when they found 13 bodies in a truck, seen as the latest explosion of violence in the battle between warring drug cartels. Soldiers found two messages at the scene in Tamaulipas state "alluding to the rivalry between criminal groups" in the northeast, the Department of Justice said in a statement overnight into Monday. Investigators say the murders are linked to Friday's discovery of 10 bodies in the town of Tampico Alto, in neighboring Veracruz state, and attacks by gunmen last week on three buses that left 16 people dead. The move was part of Calderon's national crackdown on drug cartels launched in December 2006.