By Sharyl Attkisson  | CBS News | Mar. 21, 2011

WASHINGTON - South of El Paso, Texas, on Mexico's side of the border, lies Juarez - the most dangerous city in the world. CBS News investigative correspondent Sharyl Attkisson reports ATF 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."

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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.

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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. Two of them, assault rifles, were later found at the murder scene of Border Patrol Agent Brian Terry in Arizona last December. Another gunrunning suspect under ATF surveillance was linked to the shooting of Customs Agent Jaime Zapata. And sources say many more "walked" weapons turned up at Mexican crime scenes.

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Jaquez said, "I think this incidence is probably one of the darkest days in ATF's history."

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.

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Justice Department head Eric Holder said the inspector general is investigating. "The aim of the ATF is to try to stop the flow of guns. I think they do a good job in that regard. Questions have been raised by ATF agents about the way in which some of these operations have been conducted. I think those questions have to be taken seriously, and on that basis, I've asked the inspector general to look at it."

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Jaquez is second sitting ATF agent to come forward and speak out to CBS News on the controversy.

Jaquez says one of the most difficult things for him is believing that his own agency  inadvertently put innocent lives at risk. Jaquez has family - uncles, aunts, father and sister - living in Mexico. "Any one of us could have been shot with one of those guns."

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Jaquez says he's left wondering whether runaway violence in Mexico can be partly blamed on the agency tasked with stopping it.