Air Force officials say three ballistic missile crew members in North Dakota fell asleep while holding classified launch code devices earlier this month, triggering an investigation by military and National Security Agency experts.

The probe found that the missile launch codes were not compromised, but the incident comes on the heels of a series of missteps by Air Force that has put the service under intense scrutiny.

Officials called the incident a “procedural violation,” and said the Minot Air Force-based crew had code devices that were no longer usable, since new codes had been installed in the missiles.