Wal-Mart longed to build in Elda Pineda’s alfalfa field. It was an ideal location, just off this town’s bustling main entrance and barely a mile from its ancient pyramids, which draw tourists from around the world. With its usual precision, Wal-Mart calculated it would attract 250 customers an hour if only it could put a store in Mrs. Pineda’s field.

After years of study, the town’s elected leaders had just approved a new zoning map. The leaders wanted to limit growth near the pyramids, and they considered the town’s main entrance too congested already. As a result, the 2003 zoning map prohibited commercial development on Mrs. Pineda’s field, seemingly dooming Wal-Mart’s hopes.

But 30 miles away in Mexico City, at the headquarters of Wal-Mart de Mexico, executives were not about to be thwarted by an unfavorable zoning decision. Instead, records and interviews show, they decided to undo the damage with one well-placed $52,000 bribe.

The plan was simple. The zoning map would not become law until it was published in a government newspaper. So Wal-Mart de Mexico arranged to bribe an official to change the map before it was sent to the newspaper, records and interviews show. Sure enough, when the map was published, the zoning for Mrs. Pineda’s field was redrawn to allow Wal-Mart’s store.

Wal-Mart de Mexico broke ground months later, provoking fierce opposition. Protesters decried the very idea of a Wal-Mart so close to a cultural treasure. They contended the town’s traditional public markets would be decimated, its traffic mess made worse. Months of hunger strikes and sit-ins consumed Mexico’s news media. Yet for all the scrutiny, the story of the altered map remained a secret. The store opened for Christmas 2004, affirming Wal-Mart’s emerging dominance in Mexico.

The secret held even after a former Wal-Mart de Mexico lawyer contacted Wal-Mart executives in Bentonville, Ark., and told them how Wal-Mart de Mexico routinely resorted to bribery, citing the altered map as but one example. His detailed account — he had been in charge of getting building permits throughout Mexico — raised alarms at the highest levels of Wal-Mart and prompted an internal investigation.

But as The New York Times revealed in April, Wal-Mart’s leaders shut down the investigation in 2006. They did so even though their investigators had found a wealth of evidence supporting the lawyer’s allegations. The decision meant authorities were not notified. It also meant basic questions about the nature, extent and impact of Wal-Mart de Mexico’s conduct were never asked, much less answered.

The Times has now picked up where Wal-Mart’s internal investigation was cut off, traveling to dozens of towns and cities in Mexico, gathering tens of thousands of documents related to Wal-Mart de Mexico permits, and interviewing scores of government officials and Wal-Mart employees, including 15 hours of interviews with the former lawyer, Sergio Cicero Zapata.