WASHINGTON — Just after 9 a.m. on Aug. 29, a group of U.S. airmen entered a sod-covered bunker on North Dakota's Minot Air Force Base with orders to collect a set of unarmed cruise missiles bound for a weapons graveyard. They quickly pulled out a dozen cylinders, all of which appeared identical from a cursory glance, and hauled them along Bomber Boulevard to a waiting B-52 bomber.
The airmen attached the gray missiles to the plane's wings, six on each side. After eyeballing the missiles on the right side, a flight officer signed a manifest that listed a dozen unarmed AGM-129 missiles. The officer did not notice that the six on the left contained nuclear warheads, each with the destructive power of up to 10 Hiroshima bombs.
That detail would escape notice for 36 hours, during which the missiles were flown to a Louisiana air base that had no idea nuclear warheads were coming. It was the first known flight by a nuclear-armed bomber over U.S. airspace, without special high-level authorization, in nearly 40 years.
The episode, serious enough to trigger a rare "Bent Spear" nuclear incident report that raced through the chain of command to Defense Secretary Robert M. Gates and President Bush, provoked new questions about the adequacy of U.S. nuclear weapons safeguards while the military's attention and resources are devoted to wars in Iraq and Afghanistan.
Three weeks after word of the incident leaked to the public, new details obtained by the Washington Post point to security failures at multiple levels in North Dakota and Louisiana, according to interviews with current and former U.S. officials briefed on the initial results of an Air Force investigation.
The warheads were attached to the plane in Minot without special guard for more than 15 hours, and they remained on the plane in Louisiana for nearly nine hours more before being discovered. The warheads slipped from the Air Force's nuclear safety net for more than a day without anyone's knowledge.
"I have been in the nuclear business since 1966 and am not aware of any incident more disturbing," retired Air Force Gen. Eugene Habiger, who served as U.S. Strategic Command chief from 1996 to 1998, said.
An error in a missile storage room led to missteps at every turn, as ground crews failed to notice the warheads, and as security teams and flight crew members failed to provide adequate oversight and check the cargo thoroughly. An elaborate nuclear safeguard system, nurtured during the Cold War and infused with rigorous accounting and command procedures, was utterly debased, the investigation's early results show.
The incident came on the heels of multiple warnings -- some of which went to the highest levels of the Bush administration, including the National Security Council -- of security problems at Air Force installations where nuclear weapons are kept. The risks are not that warheads might be accidentally detonated, but that sloppy procedures could leave room for theft, damage or dissemination of its toxic nuclear materials.
A former National Security Council staff member with detailed knowledge described the event as something that people in the White House "have been assured never could happen." What occurred on Aug. 29-30, the former official said, was "a breakdown at a number of levels involving flight crew, munitions, storage and tracking procedures -- faults that never were to line up on a single day."
Last fall, after 17 years in the U.S. arsenal, the Air Force's more than 400 AGM-129 cruise missiles, well known at the base as a nuclear warhead delivery system carried by B-52s, were ordered into retirement by then-Defense Secretary Donald H. Rumsfeld. Minot was told to begin shipping out the missiles to Barksdale Air Force Base near Shreveport, La., for storage.
By Aug. 29, its crews had already sent more than 200 missiles and knew the drill by heart.
The Air Force's account of what happened that day and the next was provided by multiple sources who spoke on the condition of anonymity because the government's investigation is continuing and classified.
At 9:12 a.m. local time on Aug. 29, according to the account, ground crews in two trucks entered a gated compound at Minot known as the Weapons Storage Area and drove to where the cruise missiles were stored.
A munitions custodian officer is supposed to keep track of the nuclear warheads. In the case of cruise missiles, a stamp-size window on the missile's frame allows workers to peer inside to check whether the warheads within are silver. A red ribbon or marker attached to the missile often serves as an additional warning.
The trucks hauled the missile pylons from the bunker into the bustle of normal air base traffic, onto Bomber Boulevard and M Street, before turning onto a tarmac apron where the missiles were loaded onto the B-52. The loading took eight hours because of unusual trouble attaching the pylon on the right side of the plane -- the one with the dummy warheads.