Former Mexican soldiers, police and federal agents, originally trained as an elite force of anti-drug commandos, are working as mercenaries for Mexican narcotics traffickers, bringing a new wave of drug-related killings into the United States, authorities said. Law-enforcement and intelligence officials said the well-armed gang, known as the "Zetas," is linked to hundreds of killings and dozens of kidnappings on both sides of the U.S.-Mexico border, particularly over a wide area of southeastern Texas from Laredo to Brownsville and in cities throughout Mexico.