WASHINGTON (AFP) - A computer glitch took 50 US nuclear inter-continental ballistic missiles offline for 45 minutes on Saturday, a Pentagon official told AFP. The problem was apparently a hardware malfunction, said the official, an Air Force officer with knowledge of the event who spoke on condition of anonymity. The Air Force Global Strike Command has 450 Minutemen III ICBMs in bases located in North Dakota, Montana and Wyoming. Of these, there was a communication disruption in which the Air Force lost communication with 50 ICBMs located at FE Warren Air Force base in Wyoming. The whole episode lasted three-quarters of an hour, the officer said. On Tuesday investigators discovered that similar incidents had happened at other sites more than a decade ago, so they are focusing on the hardware. It looks to be a mechanical problem with a particular part, the officer said. As soon as it happened there was a security check of every missile site by video camera and in person, and there was no apparent damage to any equipment. We have no indication of any malicious or intentional activity that would have caused this - it looks to be mechanical, the officer said. The other squadrons with 50 missiles each at Warren were unaffected, as were the 300 ICBMs at the Montana and North Dakota bases, the officer said. Aside from the 450 land-based nuclear missiles, the US military can also deliver nuclear missiles from airplanes or launch them from submarines. President Barack Obama was briefed about the event early Tuesday - after Chairman of the Joint Chiefs of Staff Admiral Mike Mullen and Defence Secretary Robert Gates, CBS News reported.