Kenneth Melson is expected to resign as acting director of the Bureau of Alcohol, Tobacco, Firearms, and Explosives in the next day or two with the agency facing scrutiny over the controversial "Fast and Furious" anti-gun trafficking operation that cost the life of a border agent, CNN's Terry Frieden reports. Melson has led the ATF since April 2009, and might be replaced by Andrew Traver, who leads the agency's field office in Chicago. Melson was "closely involved with managing" the operation.

The ATF allowed Mexican drug cartels to buy large numbers of weapons during the operation, which took place in 2009 and 2010, so it could keep tabs on gun smugglers with the hope of building a case against them. But the operation went wrong, as The Wall Street Journal's Evan Perez and Devlin Barrett report, and Republican lawmakers have been harshly critical of it:

Mr. Issa and Sen. Chuck Grassley (R., Iowa) are leading the congressional probe of Fast and Furious, which came to light after an Arizona shootout in December that killed a U.S. border agent. Two assault weapons bought in a gun shop that was part of the operation were found at the scene. The shooter and the gun used to kill the agent haven't been identified. A Mexican national is charged in the shootout.

The Journal notes that the agency "has been without a Senate-confirmed director since 2006, with both the Bush and Obama administrations unable to overcome opposition from gun-rights groups to win approval of nominees." Traver was nominated in November by President Obama to be the permanent ATF director, but the Senate has not confirmed him. The NRA says he's "demonstrated hostility" to gun rights.

[In the photo above, an ATF agent in Arizona displays an AK-47 short pistol confiscated in 2008 from a shipment headed to Mexico.]