That discovery indicates the "Fast and Furious" program, run by the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives' Phoenix office, failed to stop guns from ending up with drug gangs in Mexico, The Wall Street Journal reported.
Ostensibly, agents tracked the weapons with the intention of making bigger busts. But some lawmakers said the AFT didn't have the means to track the guns and shouldn't have initiated the "Fast and Furious" program.
Several Republican lawmakers have claimed the operation allowed suspected traffickers to purchase more than 2,500 weapons in the United States, fueling the trafficking the ATF is supposed to prevent.
One weapon used in a December shootout that killed a U.S. border agent in Arizona was traced to the ATF operation, said Sen. Chuck Grassley, R-Iowa.
Grassley and Rep. Darrell Issa, R-Calif., are leading congressional probes into the AFT operation.
U.S. Attorney General Eric Holder told a House hearing last month the Justice Department's policy is to stop weapons from being trafficked to Mexico.
"Under no circumstances should guns be allowed to be distributed in an uncontrolled manner," Holder said of the ATF program.
Firearms trafficked mostly from U.S. border states have fueled drug cartel wars that have left more than 40,000 people dead in Mexico since 2006, the report said.