Three federal firearms investigators told Congress on Wednesday that they were repeatedly ordered to step aside while gun buyers in Arizona walked away with AK-47s and other high-powered weaponry headed for Mexican drug cartels in a risky U.S. law enforcement operation that went out of control.

Rep. Darrell Issa of California said leaders of the Bureau of Alcohol, Tobacco, Firearms and Explosives were fully aware of the details of Operation Fast and Furious, which was designed to track small-time gun buyers to major weapons traffickers along the Southwest border.

At a hearing before Issa's panel, Republican Sen. Chuck Grassley of Iowa said "hundreds upon hundreds of weapons" destined for cartels in Mexico were purchased in Arizona gun shops.

The operation was designed to respond to criticism that the agency had focused on small-time gun arrests while major traffickers eluded prosecution.

In December, two assault rifles purchased by a now-indicted small-time buyer under scrutiny in Fast and Furious turned up at the scene of a shootout in Arizona where Customs and Border Protection agent Brian Terry was killed.

"We ask that if a government official made a wrong decision that they admit their error and take responsibility for his or her actions," Robert Heyer, the slain agent's cousin, told the committee. "We hope that the Bureau of Alcohol, Tobacco and Firearms is forthcoming with all information" Congress is seeking.

John Dodson, an ATF agent who feared the operation would result in deaths, told the committee that "although my instincts made me want to intervene and interdict these weapons, my supervisors directed me and my colleagues not to make any stop or arrest, but rather, to keep the straw purchaser under surveillance while allowing the guns to walk."

"Allowing loads of weapons that we knew to be destined for criminals — this was the plan," said Dodson. "It was so mandated."

ATF agent Olindo James Casa said that "on several occasions I personally requested to interdict or seize firearms, but I was always ordered to stand down and not to seize the firearms."

"We were told to just fall in line," said Casa.

ATF agent Peter Forcelli said that "when I voiced surprise and concern with this tactic ... my concerns were dismissed" by superiors.

"To allow a gun to walk is idiotic .... This was a catastrophic disaster," said Forcelli.