At least 23 people have been killed in gruesome circumstances in the Mexican city of Nuevo Laredo, on the US border.
Nine bodies with signs of torture were found hanging from a bridge.
Hours later, 14 decapitated bodies were discovered in a vehicle, police said. Their heads were found in ice boxes dumped outside the mayor's office.
Nuevo Laredo is the scene of a feud between two of Mexico's biggest drugs gangs, who are fighting for control of smuggling routes into the US.
A message left with the hanged bodies said they were members of the Gulf drugs cartel who had been killed by the rival Zetas gang.
Police said they believed the dead were members of a criminal gang, but could not confirm who was responsible.
Pictures showed the blood-stained bodies - some of them bound and gagged - hanged by the neck from the bridge over the main road from Nuevo Laredo to Monterrey.
Mexican media reports suggest the decapitations may have been carried out in retaliation for the hangings, but the authorities have not confirmed this.
Mexico's drugs cartels have long been engaged in bloody battles for control of smuggling routes into the US.
Around 50,000 people have died in drug-related violence in Mexico since 2006, when President Felipe Calderon began deploying troops to fight the cartels.
Discontent over the bloodshed has boosted support for opposition parties ahead of July's presidential election.