The so-called "Zetas" have gained notoriety for a surge in drug-related killings on the Mexican side of the border, but U.S. officials said the highly trained killers have also been operating in Dallas, Houston, San Antonio, and other U.S. cities on behalf of the Gulf drug cartel.

"These guys run like a military," said Arturo A. Fontes, an FBI special investigator for border violence. "They have their hands in everything and they have eyes and ears everywhere. I've seen how they work, and they're good at what they do. They're an impressive bunch of ruthless criminals."

Up to 10 Zetas are operating in north Texas, U.S. officials estimate, carrying out assassinations of rival drug dealers and leaders and sometimes contracting out murders to local gangs. "We're aware of the Zetas' threat to U.S. cities, and we consider it a growing threat," said Johnny Santana, a criminal investigator for the Texas Department of Criminal Justice. "We're conducting investigations into several cases statewide to establish evidence. We still don't have those links yet, but the telltale signs are there, and they point to the Zetas."

The trend marks a big change for Mexican drug cartels, which traditionally have avoided operations in the U.S. and confrontations with American law enforcement. "The Zetas are bold, ruthless and won't think twice about pulling the trigger on a cop or anyone else who gets in their way," said one former Dallas narcotics officer.

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