An ensuing investigation by military and National Security Agency experts concluded missile launch codes were not compromised -- but the incident comes on the heels of a series of missteps by the Air Force that has put the service under intense scrutiny.

Crews from Minot last summer mistakenly loaded nuclear-tipped cruise missiles under the wings of a B-52. That bomber was flown to Barksdale Air Force Base, where a bomb crew noticed what had happened.

That incident, which was considered so serious that President Bush was notified, led to command changes at Minot and was a factor in a shakeup at the top of the Air Force.

Officials called the incident involving the launch codes a "procedural violation." The Minot crew had code devices that were no longer usable, since new codes had been installed in the missiles, officials said.

The Associated Press contributed to this report