Federal security forces found two human heads Monday in Antunez, a community in the western Mexican state of Michoacan, officials said.
The heads were discovered in a trash bag in the community, which is outside the city of Paracuaro, but the victims have not been identified and the bodies have not been found, the Michoacan Attorney General's Office said.
A cardboard sign bearing a message, "This is what happens to all those who betray me," was left with the two heads.
No one has claimed responsibility for the double murder, the AG's office said.
Paracuaro is located in the Tierra Caliente region, which takes its name from the high temperatures in the area and straddles Michoacan, Guerrero and Mexico states.
The government deployed police and army troops in the Tierra Caliente region last month in an effort to stem a wave of drug-related violence.
Federal security forces took over public safety responsibilities in the region from state and municipal police amid a vacuum of authority created by drug traffickers and vigilante groups.
Officials reached an agreement with vigilante leaders under which community self-defense group members can join official security agencies and register their weapons.
Civilians began arming themselves a year ago to fight the Caballeros Templarios, a drug cartel with a large presence in Michoacan.
Los Caballeros Templarios, which was founded in December 2010 by former members of the La Familia Michoacana cartel, deals in both synthetic and natural drugs.
The gang commits murders, stages kidnappings and runs extortion rackets that target business owners and transport companies, affecting everyday life.
The cartel uses Michoacan's 270 kilometers (168 miles) of coastline to smuggle chemical drug precursors for the production of synthetic drugs into Mexico. EFE