Mexican police and soldiers have seized some 140,000 weapons since 2006, when former President Felipe Calderon deployed soldiers and federal police units to contain violence between crime groups and to hunt drug lords.

A soaring homicide rate cast a cloud over much of his six-year term. Some 120,000 homicides occurred while he was in office, and at least 60,000 of them appeared to involve criminal gangs, the study says.

Not all weapons in Mexico come from the United States, the study says, noting that “alternative sources” are suggested by the discovery of a “a wide variety of non-U.S. weaponry . . . including Soviet-era RPG-7s, Korean fragmentation grenades, M60 machine guns, Chinese TK-56s and others.”

The authors said a series of factors – such as sales from gun shows and private dealers – made their estimates, if anything, low.

Gunrunning from the United States was at the heart of a scandal that enveloped the Bureau of Alcohol, Tobacco, Firearms and Explosives, a division of the Justice Department, in 2011. In Operation Fast and Furious, ATF agents allowed “straw buyers” to purchase some 2,000 firearms. The agents’ intent was to track the guns to Mexican cartel chiefs.

But the agents lost track of some of the guns, which subsequently were used in scores of crimes that led to death or serious wounds for at least 150 Mexicans and one U.S. Border Patrol agent. Last June, the House of Representatives voted to hold Attorney General Eric Holder in criminal contempt for refusing to turn over documents related to the botched gunrunning sting.

The number of weapons involved in Fast and Furious, however, would be less than 1 percent of the number of weapons that are reaching Mexico illegally each year, if the study is accurate.

In concluding recommendations, the study suggests that purchasers of firearms in border states be barred from paying cash for the weapons. Requiring the use of checks or credit cards would “help to ensure that funds used to buy guns at legitimate establishments will not originate from illicit business activities,” it says.

The study urges more sophisticated U.S. background checks of buyers to help identify “straw purchasers” and the development of a database of seized guns in Mexico. It suggests that U.S. authorities gather more data on sales tax revenue from gun stores by county “to identify unusual activity that should be investigated.”