Paul Santorini one of the leading scientists of his time and a pupil of Einstein states unequivocally that there was a worldwide coverup going back to at least 1947 lead by the United States

In 1947 Greece’s leading scientist, Professor Paul Santorini, was supplied by the Greek Army with a team of engineers to investigate what were believed to be Russian missiles flying over Greece. Santorini’s credentials include the proximity fuse for the Hiroshima atomic bomb, two patents for the guidance system used in US Nike missiles, and a centimetric radar system. Educated at Zurich, where his physics professor was Albert Einstein (with whom he used to play violin duets!), Santorini retired as Director of the Experimental Physics Laboratory of Athens Polytechnic in 1964. On 24 February 1967 he gave a lecture to the Greek Astronautical Society, broadcast on Athens Radio, during which he revealed the results of the Greek investigation into the ghost rockets

“We soon established that they were not missiles. But, before we could do any more, the Army, after conferring with foreign officials, ordered the investigation stopped. Foreign scientists flew to Greece for secret talks with me.”

This statement was personally verified by the respected American researcher Raymond Fowler, who had written to Santorini to check on the accuracy of the newspaper quotes attributed to him following the broadcast. The professor confirmed that a ‘world blanket of secrecy’ surrounded the UFO question because, among other reasons, the authorities were unwilling to admit the existence of a force against which we had ‘no possibility of defense’. These officials were reported by newspapers to be from the United States Pentagon.

Sources: Above Top Secret by Timothy Good, Daily Telegraph 1946, Raymond Fowler UFOs: Interplanetary Visitors, and The Sydney Sun 1967