The decapitated bodies of four men were hung from a bridge in a central Mexican city ravaged by fighting between two drug lords.
A gang led by kingpin Hector Beltran Leyva said it carried out the killings in a message left with the bodies, the attorney general's office of Morelos state said.
The beheaded and mutilated bodies were hung by their feet from the bridge in Cuernavaca, a popular weekend getaway for Mexico City residents.
Cuernavaca has become a battleground for control of the Beltran Leyva cartel since its leader, Arturo Beltran Leyva, was killed there in a December shootout with marines.
Mexican authorities say the cartel split between a faction led by Hector Beltran Leyva, brother of Arturo, and another led by Edgar Valdez Villarreal, a US-born kingpin known as "the Barbie".
Authorities said the four men had been kidnapped days earlier. The family of one of the men reported the abduction to police.
In western Mexico, police found the body of a US citizen inside a car along the highway between the Pacific resorts of Acapulco and Zihuatanejo.
A report from Guerrero state police said the man was shot to death and had identification indicating he was from Georgia. Police said they had no suspects and had not determined a motive.
Mexico has seen unprecedented gang violence since President Felipe Calderon stepped up the fight against drug trafficking when he took office in December 2006, deploying thousands of troops and federal police to cartel strongholds.
Since then, more than 28,000 people have been killed in violence tied to Mexico's drug war.