In October 2000, information was provided to the Office of Inspector General concerning a security incident at Technical Area 18 (TA-18), Los Alamos National Laboratory (LANL). On September 28, 2000, at 13:48 hours Mountain Time Zone (MTZ), an alarm was activated on a vault which contains a nuclear critical mass assembly machine. It was discovered that the locking device on the reactor vault had failed, preventing the vault door from being properly secured at the conclusion of a radiological experiment the previous day, September 27, 2000. Based on this information, we initiated an inspection of this matter. The purpose of this letter report is to advise you of the results of our inspection.