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A four-volume classified study - 'Unidentified Aerial Phenomena in the UK Air Defence Region' - conducted by the UK Defence Intelligence Staff between 1997 and 2000. Released via FOIA in 2006, the report confirmed UAP sensor data is real but attributed most severe encounters to poorly understood natural atmospheric plasma formations. The study effectively closed official UK military UAP investigation while providing the most detailed Western government scientific analysis of the subject to that date.