Metropolitan Police Deputy Assistant Commissioner Martin Hewitt said the death of Gareth Williams, whose remains were found in August 2010, was "most probably" an accident - but that the facts cannot be established beyond a reasonable doubt.
Williams worked for Britain's GCHQ eavesdropping service and was attached to the overseas espionage agency MI6 when he died.
The police verdict disagrees with a coroner's inquest, which concluded that Williams had probably been killed by another person in a "criminally meditated act."
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