The story surfaced from Ray Palmer (editor of Amazing Stories), regarding a man called Fred Crisman who claimed to have actual physical evidence of a flying saucer, July 31, 1947.
Palmer passed the story onto Kenneth Arnold, who was investigating UFO reports in the Northwest. Arnold interviewed Crisman an his associate Harold Dahl who claimed they were harbour patrolmen (their first lie). Crisman reported that they had seen a doughnut-shaped craft dump piles of slaglike material on the beach of Maury Island in Puget Sound. The next morning a mysterious man in black had threatened Dahl, who claimed the man said 'I know a great deal more about this experience of yours that you will want to believe.'.
The 2 men showed Arnold the material who in turn contacted an Army Air Force intelligence officer, Lieutenant Frank Brown, who flew up from Hamilton Field in California in the company of another Air Force officer.
The 2 Air Force officers immediately recognised the material as ordinary slack but did not say so in front of Arnold due to the fact that he would feel embarrassed. While flying back to Hamilton, their B-25 caught fire and crashed, killing both officers.
Crisman and Dahl later confessed to investigators that they had made up the story. Before his death Crisman changed the Maury Incident story to that of an American Plane dropping radioactive waste instead of a UFO dropping unknown substances.
A sceptic association investigated the case, which they date to 21 June 1947 instead of 31 July 1947 (?), and gives the explanation "foundry slag, unrelated plane crash", note that Ed Ruppelt was involved in the investigation and classifies the event as "admitted hoax".