The deaths of at least a dozen people in clashes in the Mexican state of Sinaloa, as rival groups linked to the Zetas move into “Chapo Guzman’s” home territory, may be evidence that the concept of the “plaza” dominated by a single group is losing its force.

According to the Sinaloa newspaper Riodoce, the fighting started when an army patrol was ambushed on Saturday afternoon. After receiving reports of a gunfight in a rural community in the municipality of Choix, in the remote Chihuahua-Sinaloa border region, an army patrol, supported by local police, went to investigate and was attacked. They called for backup, and a helicopter which arrived on the scene also came under fire. The initial outburst of violence was followed by further fighting in an adjacent community, resulting in the deaths of several more alleged criminals.

Initially, Excelsior and other media outlets, based on local officials’ reports, put the figure at 30 dead, which would have made the incident one of the most violent firefights in recent years. However, authorities later said that only seven died, before correcting the figure to 13.

One Air Force sergeant and one municipal police officer were among those killed in Choix, with the remainder of the dead coming from the armed group that initiated the attack.

Firefights that leave dozens dead are relatively uncommon in Mexico, but not unprecedented. Last May, 30 people were killed in a running battle between criminal gangs along a highway in the Pacific state of Nayarit. In July 2010, a group of human traffickers battled against hitmen just south of the US border in Sonora, leaving 22 victims. In May 2007, gunfights between state and federal authorities in Cananea, Sonora, left 22 dead, the majority of them suspected criminals.

Choix sits within the so-called Golden Triangle, the remote portion of Mexico’s Sierra Madre Occidental where the states of Durango, Sinaloa, and Chihuahua come together. The region is the traditional home of Mexico’s marijuana and opium production, dating back to the 19th century, and Sinaloa is the home state of many of the nation’s most notorious traffickers. While its remoteness gives the groups operating there a measure of protection against the federal forces, it remains relatively to close to border crossings in Chihuahua and Sonora, which adds to its value for criminal organizations.

Initial reports said that the Choix attackers belonged to a cell linked to Vicente Carrillo Fuentes’ clan, the Beltran Leyva Organization (BLO), and the Zetas. Officials say that the mountainous region around Choix is controlled by the Sinaloa Cartel of Joaquin “El Chapo” Guzman, the primary enemy of the three groups named above. However, the cell identified by authorities has reportedly been operating in the region for several weeks, and has engaged in a series of skirmishes with Sinaloa Cartel forces.

The fighting is a manifestation of the ongoing turbulence in Sinaloa, which is ostensibly the home base of Guzman, widely considered the most powerful trafficker in Mexico. Typically, airtight control by a single capo brings a measure of peace to a given region, but Sinaloa remains one of the bloodiest states in Mexico. Indeed, in 2010, only Chihuahua had a higher murder rate or higher total number of murders than Sinaloa. Last year, just four states witnessed more killings, and Chihuahua was again the only state with a higher murder rate than Sinaloa’s.

Much of this violence stems from Guzman’s inability to definitively stamp out his rivals in the BLO, which was part of his group until 2008. Guzman is widely perceived as the winner of the years-long battle with the BLO, which has fractured into several different groups, but gunmen loyal to the Beltran Levyas, who are also from Sinaloa, have continued to operate in the Pacific state. As the reports demonstrate, they have paired up with Guzman’s principal enemies, increasing their significance.

This is not the first time this coalition of Sinaloa enemies has managed to inflict damage on Guzman’s forces. Written messages, known as “narcomantas,” taunting Guzman have repeatedly appeared in recent months in the Sinaloa city of Guasave, crowing over the murders of Guzman subordinates and claiming that the drug lord is colluding with the state government. Interestingly, the Choix gunmen were said to be from Guasave. Reports emerged late last year that the Zetas, supported by the BLO and Carrillo Fuentes, had also made their way into Culiacan, Sinaloa’s capital. These incidents suggest the continued vulnerability of Guzman’s forces, even in his own backyard.

That doesn’t mean, however, that Guzman is passively losing ground; like his enemies, he has been busy trying to expand. The violence in Choix comes just weeks after the appearance of a series of narcomantas in Nuevo Laredo, the northeastern border town that serves as one of the Zetas’ most important strongholds, heralding Guzman’s arrival into the area. One of the messages, which bore Guzman’s name and appeared alongside 14 dead bodies, accused Zeta boss Miguel Angel Treviño of using Hector Beltran Leyva’s forces to target Guzman’s people in Sinaloa.

Taken together, these incidents suggest that the concept of the “plaza” — the city or region controlled by a single drug trafficking organization, whose dominion is respected by its rivals — has grown more vulnerable. If they continue, such forays into the enemy’s turf are bound to provoke escalating acts of bloodshed.