The Zetas, meanwhile, were turning against their creator and patron, the Gulf cartel, for which the Zetas had served as muscle. They wanted a bigger piece of the drug-trafficking pie and formed an especially cruel cartel that eventually extended along Mexico's Gulf Coast into Central America.
Under leader Heriberto Lazcano, a former military trooper not yet 40 and sometimes called "The Executioner," they were more eager than other gangs to directly challenge the Mexican army, going as far as to attack military garrisons along Mexico's border with the United States in 2010. Lazcano, like Guzman, remains at large, a multimillion-dollar bounty on his head.
To finally enter Sinaloa, the Zetas essentially piggybacked on the remnants of the Beltran Leyva group, penetrating northern Sinaloa and gradually pushing farther south.
Today, the battle between the two gangs seems concentrated around the corridor running from Monterrey, Mexico's wealthiest city, to the border around Nuevo Laredo in Tamaulipas state, where Sinaloa operatives have allied with remnants of the Gulf cartel to battle back the Zetas.
Police identified the cartel henchman "El Loco" as Daniel Ramirez and have him in custody. The 49 bodies found outside Monterrey, including women, have not yet been identified.
The Sinaloa-Zeta rivalry has also spread to the Jalisco state region around Guadalajara, Mexico's second-most populous city, which one law enforcement official described as a free-for-all.
The government of President Felipe Calderon contends that today's drug-trafficking constellation is proof that its military-led offensive against cartels is working, because numerous gangs have been broken and diminished through capture and killing.
But that does not address the fact that the two cartels left standing are among the most powerful and best organized, with a virtually limitless supply of weaponry and money flowing in from the drug-consuming nation to the north, the United States.
And many here in Sinaloa suspect authorities are content to turn a blind eye to the violence, eager for the bad guys to do their dirty work, and dismissing the killing as a settling of scores, the victims as criminals.
"The state is a mute witness, waiting for them to tear each other into pieces," said Aguirre, the human rights activist. "The problem is, many innocents get caught up in it."