A US missile crew fell asleep while guarding launch codes for nuclear weapons, it has emerged.The three-member crew dozed off while in possession of the classified codes at Minot Air Force Base in North Dakota, the US air force said.The same installation was the site of a mix-up last year when nuclear weapons were loaded by mistake onto a B-52 bomber that then flew across the United States to a military base in Louisiana.That foul-up, followed by revelations that nuclear fuses had been sent to Taiwan by mistake, prompted defense secretary Robert Gates to fire the air force's civilian and military leadership last month.An internal investigation into the latest scandal found there had been a violation of procedures but no security breach, as the codes were old and had been deactivated.Commanders of the air force's 91st Missile Wing at Minot will decide whether to discipline the missile crew.