In the spring of 2014, as a team of experts was examining what ailed the U.S. nuclear force, the Air Force withheld from them the fact that it was simultaneously investigating damage to a nuclear-armed missile in its launch silo caused by three airmen.

The Air Force on Friday gave The Associated Press the first substantive description of the accident after being questioned about it by the AP for more than a year.

The accident happened May 17, 2014, at an underground launch silo containing a Minuteman 3 intercontinental ballistic missile, or ICBM. The silo, designated Juliet-07, is situated among wheat fields and wind turbines about 9 miles west of Peetz, Colorado. It is controlled by launch officers of the 320th Missile Squadron and administered by the 90th Missile Wing at F.E. Warren Air Force Base at Cheyenne, Wyoming.

The Air Force said that while three airmen were troubleshooting the missile, a "mishap" occurred, causing $1.8 million in damage to the missile. The service declined to explain the nature of the mishap, such as whether it caused physical damage, saying the information is too sensitive to be made public.

The three airmen were immediately stripped of their certification to perform nuclear weapons duty. The missile was taken offline and removed from its silo. No one was injured and the Air Force said the accident posed no risk to public safety.

More than a year later the three airmen were recertified and returned to duty.

At the time of the accident, a group of nuclear weapons experts was nearing the end of a three-month independent review of the entire U.S. nuclear force, an examination prompted in part by a series of AP stories on troubles within the force. The experts were operating on orders from then-Defense Secretary Chuck Hagel, who asked them to begin their review in March. They reported their results to him June 2.

The AP asked Lt. Col. John Sheets, spokesman for the Air Force Global Strike Command, which is responsible for the ICBM force, whether the May 17 accident had been reported to the Hagel-appointed review group. The experts were looking at a range of issues, including shortcomings in training, equipment, morale and leadership.

"No. The accident was going through the investigative process when" the review teams made their visits to ICBM bases, Sheets said. Pressed further, he said he could say no more and referred questions about this to the Pentagon, which did not immediately comment.

The Accident Investigation Board did not begin its work until Aug. 25, more than three months after the mishap. A safety investigation was begun sometime earlier. The Air Force denied an AP request for the accident investigation report in 2015 under the Freedom of Information Act.

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, said Saturday the fact that the Hagel review group was not told about the accident "raises questions about what other accidents and incidents may have been overlooked by that investigation."

On Friday evening, the AP was given a brief summary of the report. It said the Minuteman 3 missile "became nonoperational" during a diagnostic test on the evening of May 16, 2014. The next morning a "mishap crew" chief, who was not identified, "did not correctly adhere to technical guidance" during troubleshooting efforts, "subsequently damaging the missile." No further details about the damage or errors were disclosed.

The investigation report summary said there were four contributing factors to the accident, and two were identified. One was the mishap chief's failure to follow technical guidance. The other was that the mishap chief "lacked the necessary proficiency level" to anticipate the consequences of his actions during the troubleshooting.

In seeming contradiction of that second point, the Air Force said in its separate statement to the AP that the mishap team chief was properly trained for the task he was performing.

Sheets said it is possible that some or all of the three could still face disciplinary action.

The summary said the central cause of the mishap was established by "clear and convincing evidence," but the Air Force would not disclose the cause or the evidence. It said the cause is cited in the investigation report. The Air Force refused to make that public, saying the report is classified, even though the service's own policy requires the public release of accident board reports.

The amount of damage to the missile — $1.8 million, according to the Air Force — suggests that the airmen's errors might have caused physical damage, Kristensen said. If so, he said, it could have been categorized by the Air Force as a "Bent Spear" event, which is an official reporting code word for a significant nuclear weapon incident. The Air Force refused to reveal how it categorized the Juliet-07 accident.

"By keeping the details of the accident secret and providing only vague responses, the Air Force behaves as if it has something to hide and undermines public confidence in the safety of the ICBM mission," Kristensen said.

Sheets, the Global Strike Command spokesman, said Pentagon leaders were briefed on the results of the accident investigation in December. Members of Congress also were briefed, he said.