By Ioan Grillo
SAN LUIS POTOSI, Mexico (Reuters) - The most brutal drug cartel in Mexico appears to be rupturing with its hit men turning their guns on each other in a twist to the country's turf wars that threatens a new wave of bloodshed and chaos.
The killing of Zetas by Zetas, including the massacre of 14 suspected gang members on the outskirts of the central city of San Luis Potosi last month, springs from an internal feud between the cartel's supreme commander and his deputy, Mexican investigators say.
As much as the Mexican government and rival gangs may welcome discord in the Zetas' ranks, an explosion of violence ordered by its notoriously bloodthirsty leaders could be a major headache for President-elect Enrique Pena Nieto.
He takes office on December 1 and has vowed to quickly reduce the number of beheadings and mass executions seen over the past six years. But if the Zetas cartel were to break in two, it could unleash havoc as its 10,000-plus gunmen fight for control of local trafficking networks and smuggling routes.
"It's a real problem. It's like if the HIV virus mutates. Then you have to find two vaccines," said a Mexican military officer who has been battling the Zetas across the country.
Some officials tout the split as evidence of the success security forces have had in cracking down on the Zetas, from stings on their U.S. assets to assaults by Mexican marines on their urban safe houses and guerrilla-style camps.
The in-fighting has been a boon for security forces as Zetas operatives inform on their former colleagues.
And another beneficiary of the chaos is Mexico's most-wanted man, Joaquin "Shorty" Guzman, boss of the powerful Sinaloa Cartel that has been mired in a costly fight for control against Zetas across much of the country.
The ruthless Zetas have perpetrated some of the most sickening acts of Mexico's drug war and continued to expand even as rival gangs joined forces against them.
Interrogations of captured Zetas show that the split stems from disputes over shares of the cartel's criminal spoils, according to Mexican military officers and investigators.
In June, hundreds of FBI agents across the United States raided a vast horse-breeding business that they alleged belonged to the Zetas' second-in-command, Miguel Trevino. Jose Trevino, a brother of the Zetas leader, was among those arrested.
The stables received more than $1 million a month from Mexico and had more than 300 stallions, according to the FBI. One of the horses was called Number One Cartel.
"The news made other Zetas angry about how much money Trevino was taking. And with these people, anger can quickly turn to violence," the military officer told Reuters, speaking on condition of anonymity.
Rivalry between Trevino and the cartel's supreme boss, Heriberto Lazcano, has been brewing for several years as the cartel expanded far across Mexico and into Central America.
The eruption of violence helped make August the second bloodiest month since outgoing President Felipe Calderon took office in late 2006 and launched a campaign against the cartels, according to a tally compiled by newspaper Milenio. It estimated 1,341 people were killed in gang-related brutality in August.
When Zetas fight Zetas, the results are likely to be particularly gory. The cartel, which was founded by 14 army defectors in 1998, often uses military grade weaponry and is accused of the worst atrocities in Mexico's drug war.
In May, Zetas were alleged to be behind the killing of 49 people, whose bodies had their heads, arms and feet chopped off and were dumped near the city of Monterrey.
The gang has also been blamed for the murders of hundreds of people whose bodies ended up dumped in mass graves, the massacre of 72 foreign migrant workers headed to the United States, and an arson attack on a casino in Monterrey that claimed 52 lives.
When gangs turn in on themselves, killings tend to spiral as hit men have little trouble finding former comrades.
"They know where they live, where they go, what they do and who they know," said San Luis Potosi attorney general Miguel Angel Garcia.
A man who survived the recent massacre in San Luis Potosi by lying still under the pile of corpses was able to give the army the address of a Zeta leader involved in the attack.
That led to a two-hour firefight outside a university in the city as students took cover under desks. Soldiers killed three suspected Zetas members in the shootout and captured four more.
A report by Mexico's organized crime unit in January found the Zetas have a presence in 16 of Mexico's 31 states in addition to the capital, more than the oldest and wealthiest crime group, Guzman's Sinaloa Cartel.
Some experts say disputes were inevitable given the gang's rapid growth. "When a cartel expands its tentacles so much, an internal conflict is inevitable," said Mike Vigil, a former chief of international operations for the U.S. Drug Enforcement Administration.
"These types of conflicts are in the nature of organized crime, like when John Gotti fought with (Salvatore) "Sammy the Bull" Gravano," he said, referring to the late New York mobster and his former lieutenant Gravano. "There is no honor among thieves. It is all governed by money and power."
Some investigators argue that the fragmentation of the Zetas is a necessary part of destroying the criminal army.
However, others fear that the end result could be more cartels for security forces to fight.
Calderon's six-year military offensive against the drug cartels has seen 22 major traffickers and tens of thousands of their henchmen arrested or killed, causing several gangs to fragment.
In 2010, the Zetas themselves broke away from the Gulf Cartel, for whom they used to work as enforcers.
As cartels have splintered, they have often become more violent and radical, unleashing even more chaos. To date, more than 55,000 people have been killed in drug-related violence during Calderon's term.
The 14 Zetas men murdered in the van in San Luis Potosi were originally from the northern state of Coahuila and were alleged to be traffickers under the command of Trevino. They were killed by gang members close to Lazcano, investigators say.
Following the incident, there have been more than 40 other killings in San Luis Potosi and the neighboring state of Zacatecas that investigators attribute to the feud.
One body was dumped next to a note that read, "This will happen to all the traitors." In another case, a woman was shot dead after visiting a convicted Zetas operative in prison, signaling they could be targeting each others' family members.
The in-fighting may also be spreading to the northern industrial city of Monterrey, a Zetas stronghold, where a surge in violence included 33 murders in two days last month.
Rival cartels could take advantage of the Zetas feud to try to grab their assets or take sides in the dispute.
The leader of the Knights Templar cartel, a gang in the western state of Michoacan, released a video calling for an attack on the Zetas, while singling out Trevino as a target.
"We call for a common front against the Zetas, especially Z-40 Miguel Angel Trevino, a person who has sown terror and confusion in our country," Knights Templar boss Servando Gomez said from a hidden location in a video posted online.
Mexican investigators have confirmed its authenticity.
The Zetas conflict has even found its way into Mexico's famous drug ballads, which tell tales of the cartel wars. One song posted on Internet site YouTube calls Trevino "The New Judas" and accuses him of selling out other Zetas to the police.
"By betraying comrades, he got to the top," the song goes to the sound of accordions and acoustic guitars. "He knows well his objective, which is to become the leader of the Zetas."
(Additional reporting by Lizbeth Diaz and Anhai Rama; Editing by Kieran Murray and Will Dunham)