Riedel, a former CIA officer and National Security Council official and now senior fellow at the Brookings Institution, has been advising Obama.

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"It's kind of a tough mission, " the camp's senior medical officer, who was interviewed on condition of anonymity for security reasons, told CNN.

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Mr Chapman is a former deputy director of the UK Border Agency, a former Metropolitan Police officer and he has spent the last two years working as a senior security advisor for the London 2012 Olympic and Paralympic games.

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