CULIACAN, Mexico — The cartel henchman nicknamed “El Loco” was reported behind the latest atrocity in Mexico’s ever-more-depraved drug war: mutilating 49 people and piling their bodies — heads, hands and feet missing — by the side of a road leading to the U.S. border.

Authorities say he acted this month on orders from the top commanders of the brutal Zeta paramilitary force, who wanted to send a message to the long-dominant Sinaloa cartel and its allies, in a new phase of a conflict that has claimed more than 50,000 lives in less than six years.

In Mexico’s eight most violent states, the battle has essentially boiled down to a no-holds-barred fight between two cartels: the Sinaloa network, Mexico’s oldest and traditionally most powerful gang, and the newer and exceedingly vicious Zetas.

In the most brazen offensive of that war, the Zetas, smelling an opportunity, are moving steadily deeper into their enemy’s ancestral homeland here in the northwestern state of Sinaloa, long regarded as the cradle of Mexican drug-trafficking and the virtually impenetrable bastion of one of the world’s biggest drug lords, Joaquin “Chapo” Guzman.

The battle to be the last gang standing has all the terrible hallmarks of the fluid, internecine evolution of drug cartels, as alliances form, then shatter, usually with extraordinary violence, betrayal and intrigue.

Though clearly always under the sway of the Sinaloa cartel, with inexplicably ostentatious wealth, brisk Hummer sales and opulent mansions, most of Sinaloa state had remained relatively peaceful during the first years of the drug war — precisely because one group was in control and unchallenged.

That all changed when the Zetas came to town. Residents say the feud between the two cartels is starting to look like a civil war.

“It’s getting worse all the time,” said Leonel Aguirre, head of the independent Sinaloa Human Rights Defense Commission, which tallies drug-war killings and the latest scourge of forced disappearances. “Executions, decapitations, the melting of bodies.... Annihilate and terrorize. And the really bad thing is you get used to it.”

The Sinaloa cartel, for all its murderous ways, maintained something of a social base among many locals. The cartel was seen as a gang of ruthless businessmen, reluctant to “heat the plaza,” or stir things up and attract scrutiny.

The newly arrived Zetas, on the other hand, act more as an occupying force, quicker to attack, or kill, uninvolved civilians. With no connections to the local population, they’re free to act like crude hatchet men who ignore any rules.

That has deepened the fear and uncertainty here, forced hundreds of farmers to flee the suddenly violent Sierra Madre countryside in eastern Sinaloa, where most of the drugs are produced, and exposed locals to a new wave of threats of kidnapping and demands for payoffs (some delivered via email) that apparently come from the Zetas.

Zeta cells and safe houses have been detected by intelligence agencies in major cities in the state: Los Mochis in the north, Mazatlan in the south and here in the capital, Culiacan. Last month, “narco-banners,” traffickers’ homemade billboards, popped up in Sinaloa cities apparently from the Zetas and accusing government forces of siding with the Sinaloa cartel.

“We have seen inhuman acts in various parts of the country that are part of an irrational fight between two criminal organizations and their allies,” said Mexico’s top domestic security official, Interior Minister Alejandro Poire. “This is a direct struggle between [the Sinaloa cartel] and the Zetas to control routes and [territory] for their criminal enterprise.”

The arrival of the Zetas triggered a quadrupling of drug-war killings in the last four years and a jump in tactics such as extortion, kidnapping and arson.

Sinaloa, with 2.7 million residents, has half the population of El Salvador but a similar number of murders as that nation, which is also riven by violent crime. Four people a day were killed on average in the state in the first 4 1/2 months of this year, according to statistics from the state prosecutor’s office. An untold number of dead and missing are never found or recorded.

In January, Sinaloa state authorities busted a cell of kidnappers in Mazatlan and took the unusual step of announcing that the group worked on behalf of the Zetas; typically, local authorities shy away from singling out a cartel by name. Two days later, gunmen strafed the Mazatlan hardware-store chain belonging to the governor of the state (and bearing his name), killing an employee and wounding people inside.

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.”

wilkinson@latimes.com