Mexican police have found five human heads left in a sack outside a primary school in the resort city of Acapulco, the Associated Press reports.
Handwritten messages threatening three local drug traffickers were left along with the severed heads, the AP says. The heads were reported to be of men.
BBC News reports that the heads were discovered early Tuesday in a sack placed inside a wooden crate and left outside a school in the Garita neighborhood of Acapulco. Police had earlier discovered five headless bodies in and around a burned-out vehicle in another part of the city.
Acapulco, a popular tourist destination on Mexico's Pacific coast, has seen brutal violence as drug gangs fight for control of the city.
The AP says it is unclear whether the severed heads are linked to extortion threats against teachers.
The BBC reports that as a Mexican government crackdown on the country's powerful drug cartels continues, criminal organizations are increasingly turning to extortion.
Last month, dozens of teachers in Acapulco said criminal gangs had threatened them with violence if they did not hand over half of their salaries starting Oct. 1, the BBC reports.
More than 100 schools have been closed since last month after teachers and parents decided it wasn't safe enough to start classes, and teachers went on strike over security concerns.
More from GlobalPost: Mexico horror: 35 corpses dumped