Kenneth Melson is expected to resign from the Bureau of Alcohol, Tobacco, Firearms and Explosives. STORY HIGHLIGHTS NEW: The Chicago ATF field office head is to meet with Holder, sources say The sources say the resignation could occur in the coming days The operation allowed illegal gun purchases Some weapons ended up in the hands of Mexican cartels Washington (CNN) -- Kenneth Melson, acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, is expected to resign under pressure, perhaps in the next day or two, in the wake of the ongoing controversy over Operation Fast and Furious, two...