Three ballistic missile crew members fell asleep while holding classified missile launch codes two weeks ago in North Dakota, the Air Force said Thursday.
The error triggered a military investigation, which found the codes were not compromised.
But the July 12 incident comes on the heels of a series of missteps by the Air Force that already had put the service under intense scrutiny.
In this case, officials said the crew members reported themselves for the violation of procedure. The crew members apparently were leaving the control center at the end of their shift and took a set of old codes, which are typically replaced after every shift, to a rest area in the facility. Rather than proceeding back to Minot Air Force Base to dispose of the codes, they fell asleep.
Air Force officials said the deactivated codes were not at risk of being lost or stolen and that they were always contained at a secure facility.
But the Air Force did not initially make the incident public, and on Thursday the Project on Government Oversight issued a report saying the Air Force base was on "security lockdown" after discovering a "nuclear weapon launch code" missing.
The Air Force denies the base ever was locked down.
"This was just a procedural violation that we investigated," said Air Force Col. Dewey Ford, a spokesman at Patterson Air Force Base in Colorado. "We determined that there was no compromise."
The incident was serious enough, however, to prompt an investigation by the 91st Missile Wing, in conjunction with codes experts at the 20th Air Force, U.S. Strategic Command and the National Security Agency.
Ford and other Air Force officials said the Minot Air Force-based crew had code devices that were no longer usable, since new codes had been installed in the missiles.
The three crew members, who are in the 91st Missile Wing, were in the missile alert facility about 70 miles from Minot. That facility includes crew rest areas and sits above the underground control center where the actual keys can be turned to launch the ballistic missiles.There are no nuclear weapons on site.
Officials said the three officers were behind locked doors and had with them the old code components, which are large classified devices that allow the crew to communicate with the missiles. Launch codes are part of the component.
"They were awaiting to get back to base and they fell asleep," Ford said.
It delivers another blow to the beleaguered Air Force.
Missouri Rep. Ike Skelton, the Democratic chairman of the House Armed Services Committee, released a statement Thursday saying the report was "very troubling."
"There are many reasons why procedures are in place to govern strict control of our nuclear arsenal. The new Air Force leadership, when confirmed, must take decisive and urgent steps to restore the culture of respect that our strategic weapons deserve and our national security demands. This trend is unacceptable," he said.
Last month, Defense Secretary Robert Gates announced a sweeping shake-up of the Air Force leadership, blaming them for failing to fully address a series of nuclear-related mishaps.
At the time, Gates said his decisions to sack the Air Force secretary and chief of staff were based mainly on the blistering conclusions of an internal report on the mistaken shipment to Taiwan of four Air Force fusing devices for ballistic missile nuclear warheads.
He also said leadership failure ultimately was behind an August incident in which a B-52 bomber was mistakenly armed with six nuclear warheads and flown from Minot Air Force Base to Barksdale Air Force Base in Louisiana.
No one has yet to be punished in this latest Minot incident. A continuing review by Minot commanders will determine what, if any, actions will be taken against the crew members.
The investigation concluded that the codes had remained secured in their containers, which have combination locks that can only be opened by the crew. The containers remained with the crew at all times.
FOX News' Justin Fishel and The Associated Press contributed to this report.