(CNN) -- Five severed human heads were found near an elementary school in Acapulco, Mexico, in an area where some schools had already canceled classes because of lack of security.
The heads were found Tuesday inside a sack that had been placed inside a small wooden crate, the Guerrero state public security secretariat said. Officials gave no further details other than to confirm that the heads had been found.
Photos of the scene showed a board with a message that had been left with the heads. The note, in an apparently sarcastic tone, told people to thank the governor for continuing "this war."
Teachers this month held protests over threats they received, presumably from drug cartels.
The calls threatened harm if teachers did not pay a portion of their salaries to the drug gangs.
Schools that refused to pay the kickbacks would be attacked, the threats said.
Late last month, right at the beginning of the school year, teachers fled from about 75 schools after receiving threats. Administrators and other personnel also refused to go to work and many schools were left empty and padlocked from outside for two weeks.
CNN's Rafael Romo contributed to this report.