City officials suspended classes after finding the victims, most of whom had been bound and tortured. Police say some were only partially clothed.
Minutes after the grisly discovery, four other bodies were found in another empty lot in Tijuana, and two other bodies were discovered late yesterday in a lot next to a factory. Police have not determined whether the 18 killings are linked, or whether the city's drug gangs are responsible.
Tijuana is among the cities hardest hit by violence as Mexico's drug cartels battle for lucrative smuggling routes past the border to supply illegal drug users in the United States.
Nearly two years ago, President Felipe Calderon launched a nationwide battle to take back territory controlled by some of the world's most powerful drug gangs, and the cartels have responded with unprecedented violence.