Near Newell’s office is the “locker,” where confiscated guns are stored. The room is crammed with hundreds of Chinese and Eastern European AK-47s, American AR-15 rifles, shotguns, Tec-9 semiautomatic pistols, Colt .38s, Austrian Glock 9-millimeter handguns, and Fabrique Nationale 5-7 pistols; the latter are known as mata policías, or cop killers, because they fire rounds that can pierce bulletproof vests. On the floor sits a Barrett .50 caliber rifle, preferred by American military snipers because it can pick off a foe a mile away.

Almost all of these guns were nabbed crossing the border, and almost all of them, even the deadliest, are available at gun stores, sporting-goods stores, Wal-Marts, hundreds of gun shows, and tens of thousands of virtually unregulated private dealers across the U.S. “My first weekend on the job here, I recovered 30 AKs,” one of Newell’s agents, previously a detective in the Bronx, tells me. “I thought I’d seen everything, but what I see here blows my mind.” Adds Newell: “A lot of people think, ‘Well, this is Mexico’s problem.’ It’s obviously not.”

The guns move south in the same way that the drugs move north. Their flow is overseen by “gatekeepers,” transportation specialists who control “plazas,” which are border towns that serve as hubs of the drug corridors. When guns are needed in Mexico, just as when drugs are needed in the U.S., an order is called in to a gatekeeper on the U.S. side, who then subcontracts purchasers and drivers. Gatekeepers, often members of Latin American prison and street gangs that sell cartel-trafficked drugs, “own these corridors,” says Steve McCraw, director of Homeland Security in Texas. Adds Richard Valdemar, a former gang investigator with the Los Angeles County Sheriff’s Department: “If you can trust a guy to sell your 50 kilos, you can trust him to get you 50 machine guns. Guns are the stock-in-trade for bartering.”

In California, a gang known as the ­Mexican Mafia, which is run out of prisons, is tied to the Tijuana cartel. Another, called the Texas syndicate, is allied with the Gulf cartel. The MS-13 gang, which operates throughout the U.S. and Latin America, seems to work with both of them. Cash and barter are equally acceptable. In fact, with Mexico and the U.S. both cracking down on money-laundering operations, ­bartering for guns is often preferred. “One of the problems is what to do with all the bulk cash,” Newell says. “If you have your distribution network, instead of ­sending me a truck loaded with a half a million in cash, send me a truck loaded with guns.”

The preponderance of Mexican gun traces leads back to leaky border crossings—Laredo, Brownsville, and El Paso, in Texas, and Nogales in Arizona. Both states are notable for loose gun laws. Being careful to choose people without criminal records, gatekeepers often hire straw purchasers to buy the weapons and send them to stores or gun shows. A federal agent in Texas tells me of a case in which the coach of a high-school marching band had his students buy guns from pawnshops.

Adan Rodriguez’s story is a common one. Rodriguez “was a pretty typical young man in the Dallas area who made a profit from selling firearms,” says the A.T.F. agent who investigated and arrested him. At the end of 2002, while living in South Dallas, Rodriguez was approached at a local hangout by two men he didn’t know. Did he want to make some money? He didn’t have a job, so he said yes. The men didn’t tell Rodriguez their names, but several days later they left a wad of bills at his mother’s house, where he lived, along with gun-buying instructions. Early in January 2003, Rodriguez went into Ammo Depot in Mesquite, Texas; presented his driver’s license; passed the cursory background check; and walked out of the store with a 9-millimeter pistol, which he handed off.