Tijuana police said 12 bodies have been found in an empty lot next to an elementary school.
The city suspended classes after finding the bodies of 11 men and one woman just an hour before students were supposed to arrive.
Tijuana police spokeswoman Prisna Perez said the victims were bound and tortured.
Perez said four other bodies turned up in another area of the city, and two more were discovered late Sunday next to a factory, for a total of 18.
Police on Monday had not determined whether the killings were linked, and who is responsible. Tijuana is among the cities hardest hit by Mexico's drug violence.