Rivet Amber, an RC-135E, 62-4137, operating out of Shemya AFB, went down in the Bering Sea on June 5, 1969. Despite a thorough search of the area, no wreckage and no survivors were ever found. The best theory is that the plane suffered some sort of catastrophic mechanical failure, as the last radio contact from the aircraft mentioned "vibration in flight". Rivet Amber was returning to Eielson AFB for maintenance with a crew of 19 on board.[citation needed]