Congressional investigators probing the controversial "Fast and Furious" anti-gun-trafficking operation on the border with Mexico believe at least six Mexican drug cartel figures involved in gun smuggling also were paid FBI informants, officials said Saturday. The investigators have asked the FBI and the Drug Enforcement Administration for details about the alleged informants, as well as why agents at the Bureau of Alcohol, Tobacco, Firearms and Explosives, which ran the Fast and Furious operation, were not told about them. The development raises further doubts about the now-shuttered program, which was created in November 2009 in an effort to track guns across...