MINOT AIR FORCE BASE, N.D., July 25 (UPI) -- Three U.S. Air Force officers fell asleep while in control of a component with old nuclear missile launch codes, violating procedure, military officials said.

Even though the codes for nuclear intercontinental ballistic missiles had been deactivated, it was a protocol violation, prompting an investigation, CNN reported Friday.

The July 12 incident at Minot Air Force Base, N.D., is the fourth alleged mishandling in the past year.

An investigation revealed the codes weren't compromised and the crew was inside an area protected by Air Force security, the Air Force said Thursday.

The incident occurred during the changing of components used to aid secure communications between an underground missile-control facility and missile silos near Minot, said Col. Dewey Ford, a spokesman for the Air Force Space Command in Colorado. Although the new component made the old code inoperable, the old launch codes were still contained in one of the new parts.

Ford said the airmen took the component to a building above the facility, locked the component in a lockbox, then three members fell asleep. Protocol calls for at least two members to be awake while in control of the component.