In a letter Tuesday, Iowa Republican Sen. Chuck Grassley pressed Attorney General Eric Holder for details about a Bureau of Alcohol, Tobacco, Firearms and Explosives memorandum that indicates his Department of Justice may have tried to cover up the gunwalking tactics that were at the heart of Operation Fast and Furious. Gary Styers, an ATF special agent in the Lubbock, Texas field office, wrote what Grassley described as a “Fast and Furious memorandum” on Feb. 3, 2011. In it, Styers described how two investigators for Grassley’s Senate Judiciary Committee office had contacted him the day before about the Fast and...