The investigating officer believes Oscar Pistorius may be a flight risk.
"We don't want another Dewani," the officer says, referring to Shrien Dewani - the British man accused by a South African court of arranging the murder of his wife on their honeymoon there.
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.