WASHINGTON - Three ballistic missile crew members in North Dakota fell asleep while holding classified launch code devices this month, triggering an investigation by military and National Security Agency experts, the Air Force said Thursday.

The probe found that the missile launch codes were outdated and remained secure at all times. But the July 12 incident comes on the heels of a series of missteps by the Air Force that had already put the service under intense scrutiny.

"This was just a procedural violation that we investigated," said Air Force Col. Dewey Ford, a spokesman at Peterson Air Force Base in Colorado Springs, Colorado. "We determined that there was no compromise."