48 years after the USAF EC-47 was shot down over Laos, four remains out of eight airmen onboard are still missing. Originally, the USAF EC-47 was a Douglas C-47 cargo aircraft, which carried specialized electronics and flew top-secret missions. With this, a lot of people believe that the four missing crew members were actually captured and taken back to the Soviet Union. USAF EC-47 was shot down in Salavan Province, Laos, on the morning of Feb. 5, 1973.

Source: https://www.wearethemighty.com/mighty-history/mysterious-disappearance-of-an-ec-47/