MORE ON THAT ATF GUN-SMUGGLING OPERATION: The Gang That Couldn’t Sting Straight.

What did Attorney General Eric Holder know — and when did he know it?

That’s the question congressional investigators are asking — and rightly so — about the Bureau of Alcohol, Tobacco, Firearms and Explosives, which stands exposed as having perpetrated one of the most bizarre gun-sting operations imaginable.

ATF’s acting director, Kenneth Melson, is expected to walk the plank any minute now over the failed stings — Project Gunrunner and Operation Fast and Furious.

But while his ouster is a necessary first step, it can’t be the last.

Not with so many questions still unanswered. . . . Holder has ordered the Justice Department inspector general to investigate — but all available evidence points to Washington, not the local ATF bureau, as being responsible for this fiasco.

Indeed.