WASHINGTON — Three federal firearms investigators told a House committee 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, R-Calif., 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 up to major weapons traffickers along the Southwest border.

At a hearing before the Oversight and Government Reform Committee, which Issa chairs, Republican Sen. Chuck Grassley of Iowa said hundreds of weapons destined for cartels in Mexico were purchased in Arizona gun shops. One agent, John Dodson, who took his complaints to Grassley's office, estimated that 1,800 guns in Fast and Furious were unaccounted for and about two-thirds are probably in Mexico.