For the Mexican government, few things are as sensitive as an American pointing out lost territory. When Secretary of State Hillary Rodham Clinton likened Mexico’s drug trafficking last year to an insurgency, “where the narco-traffickers control certain parts of the country,” Mexican lawmakers responded with fierce condemnation.

The tensions only worsened after Carlos Pascual, the American ambassador, questioned Mexico’s crime-fighting abilities in diplomatic cables, quoting a former high-ranking Mexican official who “expressed a real concern with ‘losing’ certain regions” of the country to cartels. Mexico’s president, Felipe Calderón, was so infuriated by that cable and others that he insisted on pushing out Mr. Pascual, who resigned last month.

And yet, despite promises of help, the families and residents here say they have seen little progress in Tamaulipas. Instead, they have witnessed squabbling between top officials — Tamaulipas is governed by political rivals to the president’s party — and lackluster enforcement.

Mr. Román, the father of two missing sons, complained that government checkpoints were always in the same place and easy for criminals to avoid. Alfonso Ortega, whose brother Martín disappeared a year ago on his way to Matamoros, described a galling lack of urgency.

“The government is not moving,” Mr. Ortega said. “It’s not doing anything.”

The authorities believe the Zetas are behind the murders in San Fernando, though they have only theories about the motives: kidnappings for ransom, perhaps, or attempts at forced recruitment.

Regardless, experts say the trouble in Tamaulipas stems partly from the gang’s history. Its leaders started out as enforcers, so when they split with their former patrons in the Gulf Cartel a few years ago, the Zetas could not rely on historic ties with drug suppliers or traffickers. To thrive and expand, they branched out to other crimes, including extortion, migrant smuggling and the siphoning of oil and gas from pipelines in the area.

Image Workers unloaded a body, found in a mass grave, from a refrigerated truck last week at the morgue in Matamoros, Mexico. Credit... Alexandre Meneghini/Associated Press

Many of the gang’s early leaders served in the Mexican military, and they have used their experience to create a level of intimidation that outmatches most rivals’. No local newspaper dares to print the photos the government has issued for the 17 suspects in the latest San Fernando killings.