The bodies of nine decapitated men were found in a vacant lot in Tijuana Sunday, part of a wave of violence that claimed at least 23 lives over the weekend in this border city plagued by warring traffickers, authorities said. The heads were discovered in plastic bags near the bodies in a poor neighborhood of Tijuana, across from San Diego, Baja California state police said in a statement. Three police identification cards were also found at the site. The statement gave no motive for the killings, but they came as Mexico's drug cartels wage a bloody fight for smuggling routes and against government forces, dumping beheaded bodies onto streets, carrying out massacres and even tossing grenades into a crowd of Independence Day revelers - an attack that killed eight people in September. More than 4,000 people have died so far this year in drug-related violence in Mexico.