Reporting from Monterrey, Mexico -- In the seconds before the gunmen burst into the tiny Lozano Garza jewelry store in this city's downtown, three shoppers browsed the display cases.

An unarmed security guard sat by the door.

Then three men with assault rifles ran in, one after the other, the muzzles of their weapons ablaze.

By the time anyone reacted to the gunfire, it was too late. The four people collapsed in the barrage of bullets. One of the gunmen helped another, apparently wounded by a comrade, out of the store. Before the last killer fled, he fired final shots into a customer and the guard.

Twenty-three seconds after they came, the gunmen disappeared into the traffic of busy Francisco Madero Avenue, lined with hardware and lighting shops, taco vendors and newsstands. The page of a catalog on one case fluttered in the breeze.

The killers left the jewelry. Nor did they touch the cash register. They paid no heed to the three video cameras that recorded the entire scene.

Lying dead that afternoon of March 14, 2007, were an off-duty police commander and his wife, Benjamin Espinosa and Griselda Melendez, who apparently were shopping for a religious medallion in gratitude for a successful intestinal operation on their hospitalized infant daughter. Beyond them lay Ignacia Perez, a homemaker who lived on a former garbage dump and sold jewelry to neighbors, who paid her in weekly installments.

Sprawled near the entrance was Fernando Rodriguez, an unarmed security guard who took the $90-a-month job to save for his oldest daughter's 15th birthday celebration. He lived in terror that someone with a gun would come through that door.

A shattered peace

The victims' stories intersected in that jewelry store, but the slaughter recorded on the videotape that day looks to have been rooted in a nearby town that believed itself immune from Mexico's drug war. San Pedro Garza Garcia, southwest of Monterrey, backs up against the Sierra Madre Oriental, 15 miles and a world away from that jewelry store.

It is Latin America's wealthiest town and had long been considered one of its safest. For years, it has been the suburban escape for the industrial dynasties -- the Sada, Garza, Zambrano, Elizondo, Rivero and other families -- that manufactured steel, glass, beer and cement, and turned Monterrey into a global economic player.

Hundreds of sampetrinos, as residents are known, hold Dallas Cowboys season tickets. They shop at Maserati dealerships, and send their children to top-flight private schools. Many can navigate Houston and Aspen better than the barrios where Ignacia Perez, Fernando Rodriguez and police commander Benjamin Espinosa and his wife lived.

When Espinosa was hired as a San Pedro Garza Garcia patrol officer in 1995, it was one of the few good police jobs in Mexico. Officers there were paid about $1,000 a month.

Being a San Pedro police officer "was an honor," said Mauricio Fernandez Garza, San Pedro's mayor from 1989 to 1991. "There was a lot of recognition. They had better uniforms, of higher-quality fabric. There was a lot of training."

Fresh from two years of military service, Espinosa was aggressive and disciplined. Both his jaw and his crew cut were sharp. "He was a police officer 24 hours a day. He was very active," said Camilo Cantu, a former San Pedro chief of police.

Espinosa went to McAllen, Texas, every few months for training and target practice sponsored by U.S. law enforcement.

San Pedro police had a maximum response time of two minutes. That was possible because San Pedro never had much crime. Fernandez Garza remembers no homicides during his tenure, and in a town full of banks, only one bank robbery. No one could remember a San Pedro police officer being killed in the line of duty in those days.

The town elites kept a firm hand.

"When a daughter was marrying someone from outside," said Gilberto Marcos, a businessman and San Pedro community activist, "they'd check his bank accounts . . . to see if his money was made right. We used to look into who these people were, what family they came from. If they didn't check out, you wouldn't speak to them."