WASHINGTON (AP) — The government said Thursday that 68,000 guns recovered by Mexican 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 U.S. is responsible for arming the drug cartels plaguing its southern neighbor. 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. According to ATF, many of the guns seized in Mexico and submitted to ATF for tracing were recovered at the scenes of cartel shootings while others were seized in raids on illegal arms caches. All the recovered weapons were suspected of being used in crimes in Mexico.

At an April 2 North American summit in Washington, Mexican President Felipe Calderon said the U.S. government has not done enough to stop the flow of assault weapons and other guns from the U.S. to Mexico.

Calderon credited President Barack Obama with making an effort to reduce the gun traffic, but said Obama faces "internal problems ... from a political point of view."

There is Republican opposition in Congress and broad opposition from Republicans and gun-rights advocates elsewhere to a new assault weapons ban or other curbs on gun sales. The Obama administration says it is working to tighten inspections of border checkpoints in the absence of an assault rifle ban that expired before Obama took office.

For more than a year, 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 hasn't been part of the Republican-led probe of Fast and Furious by the House Oversight and Government Reform Committee.

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The number of all types of ATF-traced firearms manufactured in the U.S. or imported into the U.S. and later recovered in Mexico rose from 11,842 in 2007 to 14,504 in 2011, according to ATF. The figures for U.S.-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 ATF for tracing and found to have come from the U.S. 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 organizations, ATF said in a news release.

Mexico has provided ATF information on 99,691 guns. ATF determined that the source for 68,161 of the weapons was the U.S, 68 percent of the total. For the remainder, ATF was unable to determine a U.S. source or was unable to trace the request to a country of origin. The 68 percent figure is down from estimates of 90 percent in years past when Mexico was sharing less information with the U.S.

The controversial tactic of "letting guns walk" out of gun shops in the hands of suspected straw purchasers was used in Operation Fast and Furious at ATF in Phoenix in an effort to track the guns to major weapons traffickers and drug cartels 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 U.S. Two of the guns spotted at one point during Fast and Furious were later discovered at the scene of the killing of U.S. 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.

During the Obama administration, ATF has undergone a management shake-up and Attorney General Eric Holder has called Fast and Furious a flawed operation that must never be repeated.

Sen. Chuck Grassley, the top Republican on the Senate Judiciary Committee, said that thorough gun statistics are hard to come by and tricky to interpret.

"The only guns Mexico is going to submit for tracing are guns they know are from the United States, which clearly paints an incomplete picture of the firearms found in the country," Grassley said.

He said some of the guns would track back to the U.S. because of the federal government's own gun-walking operation.

Frederick Hill, a spokesman for Republicans on the House Oversight and Government Reform Committee, criticized the Justice Department over Fast and Furious.

"The Obama Justice Department's efforts to facilitate the transfer of thousands of weapons to Mexican drug cartels does not appear to have helped stop the flow of illegal weapons to Mexico," said Hill. Operation Fast and Furious lost track of 1,400 of more than 2,000 weapons agents identified as suspect purchases.

Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate Caucus on International Narcotics Control, said lax U.S. gun laws contribute to an overwhelming majority of guns recovered in Mexico that originated in the United States. Congress, she said, has been "largely silent" while Mexican drug traffickers continue to gain access to military-style firearms coming from the United States.

The period 2007 through 2011 coincides with Calderon's term as president of Mexico. Ineligible to run for re-election, Calderon made a government crackdown on warring drug cartels the hallmark of his six-year term. The election for a replacement is July 1. His center-right party has seen its election chances fall in the face of a wide perception in Mexico that the crackdown has not worked.

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 ATF when customers buy multiple high-powered rifles within a consecutive five-day period. To date, the program has been upheld in one federal court. 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.