A B-52 bomber was mistakenly armed with six nuclear warheads and flown for more than three hours across several states, prompting a US Air Force investigation and the firing of one commander, Pentagon officials said Wednesday. The plane was carrying Advanced Cruise Missiles from a base in North Dakota in the upper Midwest to a base in the southern state of Louisiana Aug. 30, said the officials, who spoke on condition of anonymity because of a Defense Department policy not to confirm information on nuclear weapons. The Air Combat Command ordered a halt in all operations Sept. 14 to review procedures, officials said. They said there was minimal risk to crews and the U.S. public because safety features in the munitions. The missiles, which are being decommissioned, were mounted onto pylons on the bomber's wings and it is unclear why the warheads had not been removed beforehand.