The US government has said that 68,000 guns recovered by its authorities in the past five years have been traced back to the United States.
The flood of tens of thousands of weapons underscores complaints from Mexico that the US is responsible for arming the drug cartels plaguing its southern neighbour.
Six years of violence between warring cartels have killed more than 47,000 people in Mexico.
The Bureau of Alcohol, Tobacco, Firearms and Explosives released its latest data covering 2007 through 2011 on Friday.
According to the ATF, many of the guns seized in Mexico and submitted to ATF for tracing were recovered at the scenes of drug-related shootings while others were seized in raids on illegal arms caches.
All the recovered weapons were suspected of being used in crimes in Mexico.
Mexico has provided the ATF information on 99,691 guns, and the ATF determined that the source for 68,161 of the weapons was the US, 68 per cent of the total.
For the remainder, ATF was unable to determine a US source or was unable to trace the request to a country of origin.
The 68 per cent figure is down from estimates of 90 per cent in years past when Mexico was sharing less information with the US.
Creating cracks to fall through
For more than a year, the ATF has been reeling from accusations that some of its agents in Arizona were ordered by superiors to step aside rather than intercept illicit loads of weapons headed for Mexico.
The Justice Department’s inspector general and Congress have been looking into the Arizona gun probe, Operation Fast and Furious.
The issue of gun control legislation has not been part of the Republican-led probe of Fast and Furious by the House Oversight and Government Reform Committee.
The number of all types of ATF-traced firearms manufactured in the US or imported into the US and later recovered in Mexico rose from 11,842 in 2007 to 14,504 in 2011, according to the ATF.
The figures for US-sourced firearms were 21,035 in 2008; 14,376 for 2009; and 6,404 in 2010.
Included in those totals, the number of rifles recovered in Mexico, submitted to the ATF for tracing and found to have come from the US rose from 4,885 in 2007 to 8,804 last year.
Mexican law enforcement officials report that certain types of rifles such as AK variants with detachable magazines are being used more frequently by drug trafficking organisations, the ATF said in a news release.
The controversial tactic of “letting guns walk” out of gun shops in the hands of suspected false purchasers was used in Operation Fast and Furious aby the ATF in Phoenix in an effort to track the guns to major weapons traffickers and drug gangs in order to make criminal cases against smuggling kingpins who had eluded prosecution for years.
But the tracking of the weapons was faulty, and many of them wound up at crime scenes in Mexico and the US Two of the guns spotted at one point during Fast and Furious were discovered later at the scene of the killing of US border agent Brian Terry.
Before Fast and Furious, ATF in Arizona had tried the gun-walking tactic in three separate investigations during the George W Bush administration, with other tracking problems and only limited success.
In the Obama administration’s efforts to slow the illicit trafficking, gun store owners in southwestern border states are suing to overturn a requirement that they report to the ATF when customers buy multiple high-powered rifles within a consecutive five-day period.
To date, the programme has been upheld in one federal court. The ATF says the reporting requirement, imposed six months ago, has led to 100 criminal investigations and the referral of 30 cases for prosecution involving 100 alleged gun trafficker defendants.