South of El Paso, Texas, on Mexico's side of the border, lies Juarez - the most dangerous city in the world. ATF [U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives] Special Agent Rene Jaquez has been stationed there for the past year, trying to keep U.S. guns from being trafficked into Mexico. That's what we do as an agency," Jaquez said. "ATF's primary mission is to make sure that we curtail gun trafficking." That's why Jaquez tells CBS News he was so alarmed to hear his own agency may have done the opposite: encouraged U.S. gun dealers to sell to suspected traffickers for Mexico's drug cartels. Apparently, ATF hoped that letting weapons "walk" onto the street - to see where they'd end up - would help them take down a cartel. Jaquez is so opposed to the strategy, he's speaking out. "You don't let guns walk. I've never let a gun walk." Yet ATF agents told us they were ordered to let thousands of weapons walk. But ATF wasn't working alone on the case known as "Fast and Furious." Documents show ATF had conference calls with "DHS" (Homeland Security). "USMS" (U.S. Marshals) and DEA. An "ICE," or Customs agent, was on ATF's Fast and Furious team. They were advised by an "AUSA," or Assistant U.S. Attorney under the Justice Department. Jaquez says he's left wondering whether runaway violence in Mexico can be partly blamed on the agency tasked with stopping it.