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It appears that the twin-engined Chinook ZD-576 flew into a hillside in dense fog. The helicopter was carrying 25 British intelligence experts from MI5, the Royal Ulster Constabulary, GCHQ and the British Army to a conference on security in Northern Ireland.
The official inquiry concluded that the crash was caused by pilot error, although this has often been disputed. Alternative theories point to problems with the RAFs upgrade program for their Chinook helicopters. In particular, other similarly upgraded helicopters had experienced problems with engine power delivery delays which may have been caused by control software.
The incident has been the subject of numerous investigations, partly because there is a dispute as to whether the crash was caused by pilot error or by mechanical failure following refit. Also the fact that the crash killed so many intelligence experts encourages speculation over official cover ups and conspiracy theories.