(Fox News) -- The killings of the US ambassador to Libya and three of his staff were likely to have been the result of a serious and continuing security breach, The Independent can reveal.

American officials believe the attack was planned, but Chris Stevens had been back in the country only a short while and the details of his visit to Benghazi, where he and his staff died, were meant to be confidential.

The US administration is now facing a crisis in Libya. Sensitive documents have gone missing from the consulate in Benghazi and the supposedly secret location of the "safe house" in the city, where the staff had retreated, came under sustained mortar attack. Other such refuges across the country are no longer deemed "safe".

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