The claim has floated around for months, circulating among academics and critics of President Felipe Calderon's military-led war on Mexican drug gangs.
It goes like this: Army and police operations that have included massive arrests, confiscation of drug shipments and numerous deadly shootouts, have left the largest and most powerful of the cartels relatively unscathed.
The so-called Sinaloa cartel, based in the drug-rich Pacific state of the same name, has been allowed to escape most of the government's firepower and carry on with its illegal business as usual, according to this theory.
Calderon had ignored such allegations until this week, when he finally felt compelled to address them.
"It is absolutely false," he said Wednesday to a reporter's question at a wide-ranging news conference. "I can assure you that this government has attacked without discrimination all criminal groups in Mexico . . . without taking into consideration whether it's the cartel of so-and-so or what's-his-name. We've fought them all."
The president went on to say that the suggestion he was playing favorites was "painful" and he questioned the motivations that lay behind it.
"These accusations are totally unfounded, false and the fruit of, in the best of cases, ignorance, if not because of ulterior interests that must be made clear," an unusually animated Calderon said. "We neither protect, nor shield, nor tolerate any criminal group in this country."
He then proceeded to tick off the names of members of the Sinaloa cartel who have been captured or killed, often using their noms de guerre: El Teo (Teodoro Garcia Simental, who allegedly melted hundreds of victims in vats of lye, captured in January), Vicente Zambada (son of one of the top leaders of the Sinaloa cartel, extradited last week to the U.S.), and so on.
Twice, Calderon interrupted reporters who were trying to ask other questions when he suddenly remembered additional Sinaloa associates who have been captured.
Those who are promoting the idea that Calderon is going light on Sinaloa gangsters point to a couple of factors. Edgardo Buscaglia, a respected academic and expert on organized crime, says arrest figures skew heavily toward the other cartels. By his calculation, of more than 53,000 people arrested in drug-trafficking cases in the three years since Calderon took office, fewer than 1,000 worked for the Sinaloa organization.
Buscaglia says that the Calderon administration may want the Sinaloa group to emerge as the main network of traffickers because it would be the easiest with which to negotiate a truce. Calderon has repeatedly said he would never negotiate with traffickers.
The government late Wednesday put out a different set of arrest figures. Of 72,000 arrested, 27% were tied to the Gulf cartel -- Sinaloa's chief rival -- and 24% to what was labeled the Pacific cartel, which included not only Sinaloa but the Beltran Leyva faction until it split off in early 2008.
The suspicions are also fueled by the near-mythic status attained by Joaquin "El Chapo" Guzman, the billionaire leader of the Sinaloa cartel. He has been on the lam since escaping from a maximum-security prison in 2001, but reports of sightings are legion.
(Most recently, a newspaper in Honduras reported that he has set up shop there in a network of luxurious, isolated ranches.) Yet he has not been caught, while some of the top leaders of other organizations have been killed or arrested.
And the "narcos" themselves have also fanned the notion that Calderon is protecting the Sinaloa organization. Rival gangs, especially the Gulf cartel and its paramilitary arm, the Zetas, have hung signs, often next to a grotesquely displayed corpse, complaining of unfair treatment.
Undoubtedly, what hurt Calderon the most and perhaps prompted him to speak out was an accusation from a member of his National Action Party. Legislator Manuel Clouthier Carrillo of Sinaloa accused the government of leaving the Sinaloa cartel virtually untouched.
His party immediately scolded him and ordered him to retract the statements or face expulsion. He has refused to back down.