The increasingly powerful Zetas are likely responsible for 50 people's killing in strongholds of the rival Sinaloa cartel in western Mexico, while the years-long drug war continues, analysts say.
The message left by the Zetas near some of the 26 corpses found Thursday in Guadalajara, Mexico's second largest city, make the targets quite clear: the Sinaloa gang and its fugitive boss, Joaquin "El Chapo" Guzman.
The messages also apparently slam an alleged alliance between Guzman and the leaders of Sinaloa state, where 24 bodies were found Wednesday, and Jalisco state, of which Guadalajara is the capital.
The killings come two months after a similar massacre in September, when 35 bodies were tipped out of trucks under a busy overpass in the eastern port of Veracruz -- an act attributed to the Zeta Killers, a group linked to Sinaloa.
"Behind the attacks in Guadalajara and Sinaloa, there would appear to be a need for revenge, fueled by the attacks in Veracruz," Dante Haro, an investigator at the University of Guadalajara, told AFP.
Haro emphasized the importance of the killings in Guadalajara, a city of more than four million people and relatively unscathed by the drug violence that has claimed some 45,000 lives since a government crackdown began in 2006.
"Jalisco state had violence rates that were lower than those in other parts of Mexico, but crime is on the rise there," Haro said.
He noted that authorities in Jalisco had captured several high-level traffickers and a Sinaloa boss was gunned down there in a security operation last year.
Those incidents stripped Guadalajara of its prior status as a neutral zone in the drug war, "where the bosses could keep their families safe," Haro said.
Raul Benitez Manuat, an expert at the National Autonomous University of Mexico's North American Research Center, said a Zetas incursion on Sinaloa turf could open a new front in a war that has ravaged cities like Tijuana and Ciudad Juarez, on the US border.
Monterrey, an industrial center in the north, has seen increased violence in recent months.
Until now, the Zetas -- set up by former army officers turned hitmen in the 1990s -- have operated mostly on the Gulf of Mexico coast in the east of the country.
For Manuat, "such a blatant operation could be a harbinger for increased violence, now on the Pacific coast."
In early October, the chief of intelligence for the US Drug Enforcement Administration, Rodney Benson, said the Sinaloa cartel had struck up an alliance against the Zetas with the Gulf cartel in the east and the La Familia cartel active in the western state of Michoacan.
The Guadalajara killings could be the first counter-attack by the Zetas, considered to be the most violent of Mexico's drug gangs and blamed for spreading extortion, kidnappings and murders.
They are believed to have been behind a casino bombing in Monterrey in August that left 52 people dead, as well as the execution of 72 illegal immigrants in August 2010.
Some 45,000 deaths have been blamed on rising drug violence since late 2006, when President Felipe Calderon launched a massive crackdown on the drug cartels involving tens of thousands of troops.