Alfredo Beltrán Leyva was arrested on January 21 in Culiacán, capital of the Mexican state of Sinaloa. The circumstances of his arrest lived up to his high standing in Mexico’s criminal underground, caught, as he was, driving a BMW S.U.V. in which federal police found eight pistols, an AK-47 assault rifle, and two suitcases containing about $900,000 in cash. Until his arrest, Beltrán Leyva was a top lieutenant in what may be the most profitable and far-reaching drug-trafficking organization in the world: the Sinaloa cartel, presided over by Joaquín Guzmán, often referred to as Mexico’s Pablo Escobar. Beltrán Leyva—known as El Mochomo after a vicious night-crawling ant—is thought by police to have been a Guzmán favorite, carrying out multiple murders while moving tons of drugs and millions of dollars for him.

The day after Beltrán Leyva’s arrest, federal police raided two mansions in Mexico City. They nabbed 11 members of his hit squad and discovered an arsenal including dozens of high-powered rifles, fragmentation grenades, thousands of rounds of ammunition, and Kevlar vests stamped FEDA. The police believe this stands for Fuerzas Especiales de Arturo, or Arturo’s Special Forces, a reference to Alfredo’s older brother, who ranks even higher in Guzmán’s organization.

One of the pistols taken from Beltrán Leyva’s truck was an American-built, silver-plated Colt .38 Super, long the preferred firearm of aesthetically inclined narco­traficantes. Originally made in the 1920s, the .38 is an iconic gun, with a sleek rectangular barrel, angled handle, and forward-thrusting look that give it a certain élan. Custom models like Beltrán Leyva’s can go for $10,000. A monogrammed, emerald-encrusted .38 Super that belonged to one infamous drug lord now resides in a Mexican museum. (View slideshow.)

Guns are nearly impossible to buy legally in Mexico, so when the Beltrán Leyva haul was brought into federal police headquarters in Mexico City, agents sent serial numbers to the American embassy. There, they were fed into eTrace, a network created by the U.S. Bureau of Alcohol, Tobacco, Firearms, and Explosives, the agency that investigates arms trafficking, and the information emerged seconds later at the A.T.F.’s National Tracing Center in Martinsburg, West Virginia. The center receives more than 800 trace requests a day. Each usually takes two weeks to process, but in an urgent situation, one can be performed in a day or less. (View an interactive look at which guns are produced where.)

The situation was urgent. A string of government assassinations was possibly in the works, according to Mexican law-enforcement officials I spoke with. Until recently, members of the Sinaloa cartel had managed to avoid the government crackdown that was devastating the rival Gulf cartel. But Mexican president Felipe Calderón now seemed to be going after the Sinaloans too, and word had come from informants that Guzmán, who’s infamous for killing politicians when he’s not buying them off, had given orders to bring the war to the capital. Beltrán Leyva, it seems, “was tasked with taking some reprisal action or took it upon himself to go out and make a hit,” an A.T.F. agent who frequently works in Mexico tells me.

To find Beltrán Leyva’s .38 Super, analysts at the tracing center sent the serial number to Colt, which produced the name of the wholesaler, who in turn dug up the location of the dealer. The pistol’s trail led back to X Caliber Guns on North Cave Creek Road in Phoenix, where it had been purchased three months earlier. From there it was smuggled over the border, probably at Nogales, Arizona. “Every gun has a story to tell,” as A.T.F. agents like to say. Beltrán Leyva’s Colt told not only its own story but also one that American and Mexican authorities and residents of the bloodstained border region know all too well—namely, that almost every gun fired in Mexico’s drug war comes from the U.S.