Agents for the Bureau of Alcohol, Tobacco, Firearms and Explosives in Arizona told the House of Representatives Oversight Committee they were told not to arrest the so-called straw buyers and instead see where the guns went.
Republicans and Democrats on the panel expressed outrage about the ATF program -- "Operation Fast and Furious" -- and demanded answers from the Obama administration about why arrests were secondary to tracking the firearms.
"We monitored as they purchased handguns, AK-47 variants and .50 caliber rifles, almost daily at times," John Dodson, an ATF special agent in Phoenix, told the committee.
"Rather than conduct any enforcement actions, we took notes, we recorded observations, we tracked movements of these individuals, we wrote reports but nothing more."
Dodson said agents were never given reasonable answers why their activities were limited.
An ATF supervisor in Phoenix, Peter Forcelli, said some tried to raise concerns with supervisors but were rebuffed.
"My concerns were dismissed," he told the committee. "I believe that these firearms will continue to turn up at crime scenes, on both sides of the border, for years to come."
The agents complained they were ordered to break off surveillance of the firearms and instead follow the original gun purchaser rather than track where the weapons went.
Drug violence and the flow of guns over the U.S. border into Mexico has developed into a major sore point between the two countries, straining diplomatic ties and leading Mexican officials to openly criticize the United States.
THOUSANDS OF GUNS TRACED BACK TO U.S.
Of the nearly 30,000 firearms recovered in 2009 and 2010 in Mexico, where gun possession is illegal, some 70 percent were determined to have come from the United States, ATF officials told lawmakers last week.
The program has renewed the political debate over tougher gun control laws.
Republicans, who largely oppose more limits, control the House and President Barack Obama's Democrats, who generally want stricter rules, control the Senate, making it unlikely that such legislation could pass before the 2012 election.
Republicans and Democrats expressed outrage at the ATF program, particularly about two weapons being found at the scene where a U.S. Border Patrol agent, Brian Terry, was killed in a shootout with illegal immigrants.
It still has not been revealed whether either of those weapons were responsible for his death.
"What we find is that people at the local level overwhelmingly objected to this program but were assured that it was approved at the highest levels," said House Oversight Committee Chairman Darrell Issa.
A report by Issa and the top Republican on the Senate Judiciary Committee, Charles Grassley, said whenever there was a shooting incident in Arizona, ATF agents feared they would be traced back to guns that were supposed to be watched.
That included the shooting in January of Democratic Representative Gabrielle Giffords, who was gravely wounded. That gun has not been linked to the ATF program.
"The allegations that have been made are very troubling and new information we have obtained raises additional concerns about the roles of various actors involved in these incidents," said Elijah Cummings, the top Democrat on the panel.
Republicans on the panel have demanded documents and information about the program from the Justice Department, which includes the ATF, but the Obama administration has resisted pending its own investigation and prosecutions.
The Justice Department's inspector general is looking into any impropriety in the program. Prosecutors have also brought charges related to the death of the border patrol agent.