Citing ATF data, investigator Jess T. Ford said that over the past three years, more than 90 percent of the firearms traced after being seized in Mexico have come from the U.S.
The figure is slightly less over a five-year period.
"While it is impossible to know how many firearms are illegally trafficked into Mexico in a given year, over 20,000, or around 87 percent, of firearms seized
by Mexican authorities and traced
over the past five years originated in the United States," the GAO's Ford said in testimony prepared for Congress.