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.

Across Tijuana on Sunday, attacks by gunmen killed five people in addition to the nine beheaded bodies.

State police said nine more people were killed in attacks on Saturday. In one, gunmen killed a 4-year-old child in an attack on a grocery store.

Baja California has suffered a rising wave of homicides, which officials blame on a struggle between rival cells of the Arellano-Felix drug cartel.

The Arellano Felix cartel emerged in the 1980s as a drug trafficking powerhouse across the U.S. border from San Diego, but has been weakened in recent years as leaders were killed or captured.

Last month, police arrested Eduardo Arellano Felix, the alleged leader of the cartel. Authorities say his nephew, Luis Fernando Sanchez Arellano, has taken over the cartel's operations and is fighting contenders.

President Felipe Calderon on Sunday vowed his government would never negotiate with drug lords no matter how much the violence escalates.

Since taking office on Dec. 1, 2006, Calderon has sent more than 20,000 soldiers to battle drug trafficking across Mexico, helping to seize of 70 tons of cocaine and 3,700 tons of marijuana, he said.

"We know that the results are far from what society demands, but that's why we'll keep fighting these criminals across the country," Calderon told a meeting on Sunday marking his first two years in office.