WASHINGTON (AP) — Attorney General Eric Holder says a flawed investigation of arms traffickers called Operation Fast and Furious never should have occurred and must never happen again.On the eve of an appearance before the Senate Judiciary Committee where he faces tough questioning by Republicans, the attorney general says the mistakes in Fast and Furious should not deter the government from disrupting the dangerous flow of firearms along the Southwest border.The probe employed a now-controversial investigative tactic known as gun-walking. Several agents said they were ordered to let suspected straw buyers walk away from Phoenix-area gun shops with AK-47s and other weapons believed headed for Mexican drug cartels, rather than arrest and seize them there. The goal was to track the guns to trafficking ring leaders, who had long escaped prosecution.