Two Air Force ICBM launch officers at Malmstrom Air Force Base in Montana have been implicated in an illegal-narcotics investigation, a U.S. defense official said Thursday, adding to the list of troubles uncovered by Associated Press reporting in the past year on the country's nuclear force:

—In April, 19 missile crew members in the 91st Missile Wing at Minot, N.D., were deemed temporarily unfit for duty and given weeks of remedial training. The wing's deputy commander of operations complained of "rot" in the force. Later, the officer in charge of the 91st's missile crew training and proficiency was relieved of duty.

—In August, the 341st Missile Wing at Malmstrom failed a safety and security inspection. Nine days later the officer in charge of security forces there was relieved of duty. In October the unit passed a do-over test.

—On Oct. 11, the Air Force fired Maj. Gen. Michael Carey, commander of the 20th Air Force, which is responsible for the entire Minuteman 3 missile force, amid an investigation of an alcohol-related complaint. This happened two days after a Navy admiral who was second-in-command at U.S. Strategic Command, the military's main nuclear war-fighting command, was relieved of duty amid a gambling-related investigation.

—The AP reported that twice last year the Air Force has punished officers involved in separate incidents of opening the blast door of their launch control center while one of the two launch officers was asleep, in violation of Air Force rules.

—In November, the Air Force chief of staff, Gen. Mark Welsh, disclosed that as a result of the Carey firing, the Air Force would take a closer look at the background of candidates for general officer-level nuclear command jobs.

—Also in November, the AP reported that key members of the Air Force's nuclear missile force are feeling "burnout" from what they see as exhausting, unrewarding and stressful work. The finding, in an unpublished RAND Corp. study provided to the AP in draft form, also cited heightened levels of misconduct such as spousal abuse and said court-martial rates in the nuclear missile force in 2011 and 2012 were more than twice as high as in the overall Air Force.