Zetas are former Mexican military soldiers trained by the USA.
They operate in Mexico and have according to law enforcement agencies have been known to come to Texas and execute hits on rivals.

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Translated from here - http://www.eluniversal.com.mx/nacion/159612.html

Following a meeting held this Sunday, The Zeta became a new cartel in Mexico at independence two of the leaders of the hired assassins formerly worked for the Gulf cartel, which Mexican drug trafficking organizations establish a new map, said information intelligence from the U.S. government.
A senior official of the Drug Enforcement Agency (DEA, for its acronym in English), who requested anonymity because there is an ongoing investigation on the Zeta and is still classified information, confirmed that seek arrest warrants against Heriberto Lazcano and Miguel Trevino Morales, leaders of the new banda, on charges of drug trafficking and murders committed in the territory of the United States.

"The meeting this Sunday on the Mexican side simply materialize an idea where they had been working since last year, and now with the pressure of the government of Mexico on The Federation, but especially against the Sinaloa cartel, arrived The opportunity for Zeta, which is doubly dangerous because they can overcome any cartel known far and represent a real national security problem for Mexico and the United States, "he said.

The U.S. government, he said, acknowledging that the Zeta are known for their violent methods and intimidation and have also been working closely with corrupt law enforcement officials.

Report of the DEA and the Central Intelligence El Paso said that the meeting took place in Torreon this Sunday was chaired by Heriberto Lazcano, El Lazca, who assured its partners that contacts with groups in Colombia allow them to operate traffic of cocaine in the corridor of the border between Mexico and the United States, especially between Tamaulipas and Texas, and in Tabasco, part of the Yucatan Peninsula and the area between Guerrero and Michoacan.

In addition, the specific information that intelligence agencies from the United States have monitored previous meetings of Lazcano and Miguel Trevino Morales with contacts in U.S. territory with those alliances, and those who ensure the delivery of drugs by having an armed wing to them allows eliminate their enemies, because their intention is to position itself as the main cartel in Mexico.

According to official U.S. National Intelligence Centre on Drugs, The Zeta have a presence in the United States less structured than in Mexico, but already have managers in the border area, leaving Mexican gangs distributing drugs, so that the cartel might be more sophisticated and violent.

However, added, the people of Lazcano and Trevino still operates for the Gulf cartel, which provides a new confrontation due to the Zeta loyal to Osiel Cardenas Guillen, leader of the Gulf cartel, have trained both in U.S. and Mexico to new generations of young people hired assassins. "

Zeta are former soldiers of the Mexican Army special forces and maintain proficiency in the use of heavy weapons, military tactics specialized, sophisticated communication equipment, gathering intelligence and espionage techniques," says the report on the national assessment of Threat of Drug 2008.

With earnings until they reach 23 billion dollars annually by the narcotics trade in the United States, it is clear to know why the Mexican cartels maintain a war for control of the business, said information from the Centre National Drug Intelligence.

In addition to smuggle 90% of the cocaine that arrives in the United States, Mexican cartels are entering a year more than 9 thousand tons of marijuana produced in Mexico, 17 tons of heroin and an unspecified amount of methamphetamine, leading well into the business of drug trafficking U.S. territory, pointing.