28 September 2011 Last updated at 04:55 ET BBC
Mexican police have found five decomposing heads left in a sack outside a primary school in Acapulco.
Handwritten messages were also found, reportedly threatening the state governor as well as local drug lords.
It was not clear if the discovery of the heads and five decapitated bodies elsewhere in the city was linked to extortion threats against teachers.
Dozens of schools have been closed since last month after teachers went on strike over security concerns.
Police were called to a street in the Garita neighbourhood of Acapulco on Tuesday morning.
There they found a sack inside a wooden crate placed near the school, officers said.
Inside were the heads of five men, as well as the threatening messages.
Police had earlier discovered five headless bodies in another part of the city, left either inside or near a burned-out vehicle.
Acapulco, on Mexico's Pacific coast, has seen several episodes of gruesome violence as drug gangs fight for control of the resort city.
The new school year has been disrupted
But as the government crackdown on cartels continues, criminal organisations here and in other parts of Mexico are fracturing and increasingly turning to extortion.
Last month, as the new school year began, dozens of teachers in Acapulco said criminal gangs had threatened them with violence if they did not hand over half their salaries from 1 October.
They and colleagues have since been on strike, leading to the closure of more than 100 schools.
Guerrero State Governor Angel Aguirre has promised a series of measures, including increased police patrols and the installation of security cameras and panic buttons in schools.
But teachers say they still fear for their own and pupils' safety.
One striking teacher told the BBC that although they welcomed the governor's proposals, they could not expect the situation to improve overnight.
Is Drug Legalization the Only Answer?