Officially, Mexico maintains far loftier goals. The 1917 constitution requires that the penal system be organized “on the basis of labor, training and education as a means of social readjustment.” But the rhetoric has never matched reality, and now the correctional system is widely described as a disgrace. Since 1992, when drug traffic began to shift toward Mexico from the Caribbean, the country’s prison population has nearly tripled, to about 240,000 inmates. While the government has done little to shore up a notoriously weak justice system, sentences have become longer and jails have become increasingly packed as officials send more soldiers and police officers into the streets to attack drug gangs.

Kingpins are usually extradited to the United States. Midlevel capos come and go. Those left behind tend to be repeat criminals and low-level offenders. The most reliable surveys of Mexico’s prison system — conducted every few years by two social scientists, Elena Azaola and Marcelo Bergman — have found that a majority of Mexico’s inmates are incarcerated for stealing items worth less than $400 or for selling small quantities of drugs. Many claim to be innocent. Some no doubt are. A majority of Mexican inmates did not have a lawyer present when they made statements to the police. “It’s usually the poor and the last links of the chain,” Bergman told me. “They’re the ones who are getting caught.”

Saltillo, a sprawling industrial city a few hours south of Laredo, sits in a wide valley surrounded by toothlike mountain ridges. It once served as the capital of a vast desert region that included most of Texas. These days its earlier ambitions can be seen only in the 18th-century cathedral that rises over downtown with its hulking steeple. The other obvious landmarks are smokestacks from the factories pumping out heaters and toilets, diesel engines and Chrysler trucks ­ — and the pink guard towers of the Cereso, a campus of concrete, steel and earth nearly half a mile wide, on the eastern edge of the city.

The Zetas are relatively new arrivals to the area, having worked as enforcers for the Gulf Cartel until splintering off around 2007. The gang’s founders were mostly corrupt former soldiers who appear to have chosen the state of Coahuila, with Saltillo as its capital, because it sits between the Pacific smuggling routes controlled by the Sinaloa Cartel and the eastern coast controlled by their former employers. The area has the added advantage of being close to the U.S. border, and for all these reasons, it is now a major operating base. When Heriberto Lazcano Lazcano, one of the Zetas’ top leaders, was killed by Mexican marines in October, the shootout occurred in Coahuila (as did the theft of his body while the government was still trying to confirm his identity).

At first, the Zetas had no relationship to Colombian cocaine suppliers, so they amassed power through creativity and intimidation, using extortion, kidnapping, migrant trafficking and the theft of resources, like coal and oil, to help supplement their smuggling income. In the process, they have taken public brutality to new levels. In March, three days after five bodies — naked and wrapped in sheets like mummies — were found on a Saltillo street, the local paper ran an editorial declaring it would stop publishing information on organized crime because “there are no security guarantees for the full practice of journalism.” Prison officials gave in earlier than that. Sixteen months ago, the Saltillo Cereso warden who was in charge when the Zetas took over was shot 10 times in his car, in broad daylight, as a school let out a few yards away.

Since then, the prison has become just another revenue source. New arrivals are often little more than hostages, like M., trapped inside and forced to wait and see if their parents can find the ransom money to keep them alive. Access to work and education, or even food and soap, has also been monetized. Prisoners selling candy for the pittance they need to survive must pay the Zetas a tax of 100 pesos a week ($8.25). Jobs in carpentry shops, at prison bodegas or in factories inside the prison all come with a fee, as do materials like lumber, which the Zetas provide for triple the going price. Drugs and alcohol, sold on Saturday nights, cost about what they do on the outside, though inmates must pay $1 for the right to exit their cells and buy them. For as long as the comandante is in charge, all that money flows to him, a mysterious figure believed to be in his 30s, whom no one dares name. Several inmates told me that he was rarely seen but universally feared, running the operation from his comfortable quarters in the conjugal-visits building, where the rooms lack bars but not air-conditioning, which was installed by the Zetas themselves. Right next door sits Coogan’s Catholic chapel.

Coogan did not come to Mexico to save anyone. He first arrived in Coahuila in 1988 through a job with the campus ministry of his alma mater, Fordham University. The second oldest of 14 children born to a corporate lawyer with a degree from Harvard Law School, Coogan spoke no Spanish and had never traveled outside the United States. But he appreciated the sense of community he found in Mexico and the effort to survive collectively. “It was like the Brooklyn I grew up in, with people out in the street,” he said. “You go for a walk and you see your neighbors. You talk. I found that incredibly appealing.”