That the Zetas would dare to invade Guzman's home turf reflects their own sense of ascending strength and a perceived vulnerability on the part of the kingpin, whose capture is considered the top prize in the drug war, especially in an election year. As the on-again, off-again hunt for Guzman heats up, there have been reports of grumbling among Sinaloa lieutenants; U.S. law enforcement agents believe some Sinaloa cartel leaders increasingly see Guzman as a liability.
At the same time, however, Guzman's forces have roared into areas once dominated by the Zetas, such as eastern state of Veracruz, where four journalists were killed (two of them dismembered) in a recent week. Sinaloa operatives may be behind a group called the Mata-Zetas ("Zeta-killers") who signaled their debut by dumping 35 mangled bodies into the center of Veracruz city last summer.
Guzman traces his rise to the 1990s; he had split with some of the original older-generation traffickers and taken up with the Beltran Leyva brothers. All hailed from the same sparse rural section of Sinaloa around Badiraguato, in the Sierra Madre foothills. The Beltran Leyvas tended to the northern part of Sinaloa state, including the transport routes that led northward through neighboring Sonora state and into the U.S.
Things began to unravel in 2008. Alfredo Beltran Leyva, the group's top commander, was captured by the army that year, and his successor, Arturo Beltran Leyva, was killed in a shootout with Mexican marines in 2009. Some in the Beltran Leyva faction suspected that Guzman had ratted them out; the faction split off from the Sinaloa cartel and fragmented amid a bruising power struggle.
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.
The Zetas were built with deserters from the Mexican army's elite airborne special forces and then augmented by hardened commandos fromGuatemala'sKaibiles, a notorious military unit trained by U.S. advisors.
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."