The bodies of 11 men and a woman were found dumped in an empty lot next to a Tijuana elementary school Monday morning, an hour before children were scheduled to arrive.

City officials suspended classes after finding the victims, most of whom had been bound and tortured. Some were only partially clothed, said Prisna Perez, state police spokeswoman in Tijuana.

Minutes after the grisly discovery, four other bodies were found in another empty lot in Tijuana, and two other bodies were discovered late Sunday in a lot next to a factory.

Investigators believe 16 of the victims were killed by warring drug gangs. The other two were victims of street crime, said Jose Manuel Yepiz, spokesman for the Baja California attorney general's office.

Victor Clark Alfaro, director of the Binational Center for Human Rights, said the killings come as the Arellano Felix cartel suffers internal strife and seeks to fend off Joaquin "El Chapo" Guzman's push into the region.

Yepiz said the 12 bodies were discovered next to a message written on a white piece of cardboard that read, "This is going to happen to all of those who are with 'The Engineer' for being blabbermouths."

He said police also found a message with the four bodies found in an empty lot in a residential area that read, "I'm not a traitor but I don't like to be in the company of cowards."

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.