Now, the sense of horror has returned. Last week, officials said they had discovered mass graves on another secluded ranch there. Some 88 dead have been unearthed so far as forensics teams continue to dig for others.

That history has repeated itself in San Fernando may be no surprise. It fits the profile of a lawless county: Fewer than 60,000 people live in an expanse of hundreds of square miles; it lies along a major highway that snakes up to the border with Texas, making it a good place to ferry U.S.-bound illegal drugs. Hot flatlands of sorghum stretch out for acres from the main town, the area's cash crop whose isolated ranches also serve as the gathering points of organized crime groups. A small municipal police station usually has four officers on patrol.

Some say San Fernando is out of the reach of the law. Rosa María Lozano de Anda, an art teacher in the town, said her cousin was kidnapped in February and hasn't been heard from since. Her reaction after the bodies were found shows the hardened psychology among some countryside dwellers, where violence has become the norm. "What did I feel when I heard? The truth is it made me happy: Finally those whose loved ones had disappeared knew that they were dead. They'll bury them and in 24 hours they'll get over it."

The August massacre claimed no victims from San Fernando. The 72 dead were immigrants traveling north to the U.S. from places like Honduras, Guatemala, Ecuador and Brazil. Authorities said the travelers were apprehended by Los Zetas, a criminal group that mixes drug trafficking with other rackets like extortion and kidnapping for ransom.

One immigrant who managed to escape told investigators that victims had been given the choice of paying money to be released, or joining Los Zetas as drug runners and hit men. But they were killed instead, the reasons for which are still unclear. Days later, the two investigators who had been assigned to the case were also killed.

This time around, however, the victims were likely entirely Mexican. Officials said the dead included a group who were kidnapped in March on public buses by Los Zetas. Most of the bodies were transported north last week to the city of Matamoros, the nearest major city, for identification as weary families followed them to hear if their loved ones were among the dead. So dangerous are the roadways that this past weekend several Mexican bus lines said they had suspended bus service through Tamaulipas state, partly because of the San Fernando incidents.

In the months between the two massacres, San Fernando and surrounding Tamaulipas state have become a no-man's land. Piles of bullet-ridden bodies have been found along the county's roadsides, in various states of decay. Shootouts between sport-utility vehicles—cars popular among drug traffickers—have erupted nearly on a daily basis. On most days, when twilight settles on the area, the streets of San Fernando, the county seat, empty as residents give way to the criminals who own the highways and country roads, residents say.

Locals have turned to their churches for guidance, but even Catholic priests appeared guarded about the situation. At Mass, pastors are "at risk if they spoke about the crimes," said a man at a San Fernando church recently, who refused to give his name. "People have learned to read in between the lines at sermons."

The man emailed a reporter pictures he had taken in recent months of abandoned sport-utility vehicles that are now a common sight along the roads. One snapshot showed the remains of a truck beside a sorghum field, its frame barely standing. In another, a bullet-pocked Ford sits with its tires missing, telltale signs of a fierce firefight.

Authorities, including the state prosecutor, are not discussing the latest massacre, wary of what happened to the investigators last time. The town mayor didn't respond to written questions nor did other Tamaulipas state officials. Many in the state government have been keeping low profiles since last year when the leading candidate for governor was shot dead on a highway days before his election.

Anxiety rules even in the San Fernando police office. On a recent day it was empty, save for a receptionist. It has been attacked twice by armed commandos, the last time in October when men sprayed the building with bullets.

"I hid under my desk," the receptionist said. "I was the only one here."