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”.

Some of the missing papers from the consulate are said to list names of Libyans who are working with Americans, putting them potentially at risk from extremist groups, while some of the other documents are said to relate to oil contracts.

According to senior diplomatic sources, the US State Department had credible information 48 hours before mobs charged the consulate in Benghazi, and the embassy in Cairo, that American missions may be targeted, but no warnings were given for diplomats to go on high alert and “lockdown”, under which movement is severely restricted.

There is growing belief that the attack was in revenge for the killing in a drone strike in Pakistan of Mohammed Hassan Qaed, an al-Qa’ida operative who was, as his nom-de-guerre Abu Yahya al-Libi suggests, from Libya, and timed for the anniversary of the 11 September attacks.