The overwhelming majority of weapons fuelling Mexico's drugs war are being sold in the US, where gun dealers are responsible for supplying at least 60,000 of the illegally trafficked firearms seized by the authorities since 2006.
Cartels cite Texas, which has some of the most relaxed gun laws in the developed world, as their shopping destination of choice. The state is home to eight of their favourite 12 gun dealers, according to a year-long investigation published yesterday by The Washington Post.
Four of the most widely used outlets are owned by Bill Carter, a former president of the local gun dealers' association who has lobbied extensively against stricter firearms laws. His stores have sold at least 155 of the guns seized by Mexico's police and army in the past two years. On one occasion, a man bought 15 .223-calibre sniper rifles, worth $9,000, from one store on a single day.
Efforts to stop the flood of arms over the border, detailed by the newspaper, paint a chilling portrait of the price of Americans' right to bear arms. More than 28,000 people have been killed in violence in Mexico since 2006, when the government launched its "war" on drugs. Most of them died from weapons that were sold in the US.
In Texas, 3,800 gun dealers are freely permitted to sell high-powered equipment such as AR-15s, AK-47s, and armour-piercing .50-calibre machine guns. These now account for between 80 and 90 per cent of the weapons held by drug cartels.
The US has long been accused of selling guns smuggled south, but the extent of the problem has until now been unclear, thanks to a law passed by Congress in 2003, which means the names of dealers whose guns are found in Mexico remain confidential.
The Washington Post got round the law by reviewing hundreds of court documents and interviewing dozens of Mexican officials and US law-enforcement personnel.
In just one of the criminal conspiracies it uncovered, a gang of 23 traffickers bought more than 335 firearms, including 251 rifles, from 10 US dealers, over a few weeks. One member of the gang bought 14 AK-47s in one day from a single dealer. He paid cash. About one-third of those weapons were subsequently traced to incidents in Mexico involving 63 deaths, including a police massacre in Acapulco in which men disguised as soldiers killed seven people during a raid on the city's Attorney General's office. None of the gun dealers responsible was charged with selling firearms illegally. The US authorities rarely bring criminal cases in Texas, the newspaper said, because laws backed by the gun lobby make it difficult to have charges stick. Since 2006, only two gun dealers there have lost their licence.
The problems authorities face are illustrated by the case of George Iknadosian, who owned a gun store called X-Caliber and was caught on tape telling undercover agents posing as gunrunners how to sneak weapons across the border, advising them to cross on weekends, when border agents might be off fishing. Although he sold 47 guns later linked to crimes in Mexico, charges against him were thrown out by a judge.
Mr Carter has yet to be successfully charged with making an illegal sale. In a newspaper advertisement in April, he joked: "Why all the talk about guns going south when so many drugs are coming north that our cows along the interstate are gettin' high off the fumes!"
The National Rifle Association denies US weapons are responsible for most of the deaths south of the border.