Records compiled by the Bureau of Alcohol, Tobacco, Firearms and Explosives indicate that 90 percent of the weapons seized from Mexican drug cartels were sourced in states as far north as Michigan and Connecticut as well as non-border states such as Florida and the South Carolina.
USA Today said Thursday that the investigation into a major gun battle in Tijuana last year took ATF agents to Seattle, Philadelphia, Denver and San Francisco.
"Every time we open one of these cases, we are learning something new," said William Hoover of the ATF.
The newspaper said the flow of guns into Mexico has become alarmingly efficient with some weapons seized in Mexico within days of being purchased from U.S. dealers.
Legislation introduced Wednesday in Washington would provide $379 million for enhanced anti-smuggling efforts, including inspections of vehicles crossing into Mexico and new investigations by the ATF in the border area.