Two severed heads were dumped in front of a bank in the western Mexican state of Michoacán on Monday, days before a planned visit by Enrique Peña Nieto, the president, to the volatile region where he has launched a major security offensive.
Though there was no confirmation of the identity of the victims, officials believe the murders in the town of Parácuaro were the work of the Knights Templar cartel seeking to dissuade potential informants.
(Read more: Mexico's economy may surprise you)
Valor por Michoacán, a local group that monitors the security situation, posted on Twitter that the victims were found with a sign reading "for all those who switched sides". An official who asked not to be named confirmed that the message was a warning to potential traitors about what could happen to them.
Mr Peña Nieto, who is due to travel to Michoacán this week although no date has yet been announced, deployed some 9,300 federal police and troops to the so-called Tierra Caliente (Hot Lands) region of Michoacánlast month to wrest back control of a state with an important Pacific port, and where so-called self-defense groups of armed vigilantes had taken the law into their own hands to fight the Templars.
It was the third time in under a year that the president had sent federal forces to the state, where the cartel has branched out from drug trafficking and extortion to run a lucrative line in smuggling iron ore to China. The move appeared to be paying early dividends, including the capture of the alleged number two in the Templar hierarchy, and a deal with self-defense groups to legalize and join rural police forces.
Nonetheless, the ability of criminal gangs to slip unnoticed into Parácuaro and dump the heads in a plastic bag in front of the bank at the entrance to the town, despite the heavy police and army presence in the region, underlines how difficult it will be to tame the Templars.
Two self-defense group members also admitted last week to having being armed by a rival drug gang, the government says, highlighting the parallel challenges of reining in vigilante groups that sprang up last February.
(Read more: Fed tapering good for Mexico: Finance Minister)
"We always took for granted that this kind of thing would happen – it's a complex conflict," said the official, referring to the severed heads, a gruesome tactic used by drug lords in the past to send warnings.
The government last week reported skirmishes with armed men and the capture of an arms haul, a safe, and drugs as well as the destruction of a drugs lab and marijuana plantations.