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I have always wondered why the accident aircraft was only just over half-full.
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There has been speculation in the past that some elements of the American community in the UK had been tipped off about a possible attack on a US airliner out of London just before the Christmas holidays that year. For the first time some Americans, especially in the diplomatic/governmental corps, had been quietly advised to use non-US airlines to go home with for the Christmas/New Year holiday.
Now, I stress I am not endorsing this theory myself - but it is something that I heard both at the time and occasionally in the years since.