A review of cases over the last two years shows a pattern: the drug cartels hire people in need of cash with no criminal records to buy guns from legal sources, often just one or two at a time.

Once the smugglers have amassed a cache of weapons, they drive them across the border in small batches, stuffed inside spare tires, fastened to undercarriages with zip ties or bubble-wrapped and tucked into vehicle panels. In some cases, the drug traffickers and gun smugglers are linked.

On a recent evening in Reynosa, a border town, a Mexican army patrol found an abandoned farmhouse that had been used by drug traffickers. Hidden deep in the brush outside was a plastic barrel filled with guns. The authorities believe that the traffickers were taking drugs to the United States and using the money to return with guns.

The cartels also employ spies to keep track of the sporadic efforts of the Mexican military to search cars, law enforcement authorities say. Because there is no computerized national gun registry, agents say, tracking guns relies on a paper trail. Agents must contact the manufacturer or importer with a make and a serial number and work their way down the supply chain by telephone or on foot.

At the retail level, records of gun sales remain in the hands of the dealers. Agents can request to see them only if a gun is recovered in a crime or during periodic audits. By law, those audits can be done only once a year, and, in practice, most dealers face such a review once every three to six years, because auditors are stretched thin.

The record keeping is not always perfect. In trying to track guns confiscated in Mexico last year, agents found that one in five of the guns could not be traced because the dealers had no record of the sale or had gone out of business and the records had been lost.

Even when the original legal buyer is located, a gun owner in many states, can legally say “I lost it” or “I sold it to someone I do not know.”