The killings of the US ambassador to Libya and three of his staff were likely to have been the result of a security breach, i can reveal.

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, but American officials believe the attack was planned. 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 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 would be targeted, but no warnings were given for diplomats to go on high alert and "lockdown", under which movement is severely restricted.

Officials are increasingly convinced the ferocious nature of the Benghazi attack, in which rocket-propelled grenades were used, indicated it was not the result of spontaneous anger due to the video, called Innocence of Muslims. Patrick Kennedy, Under Secretary at the State Department, said he was convinced the assault was planned because of its extensive nature and the proliferation of weapons.

There is growing belief that the attack was in revenge for the killing in a drone strike in Pakistan of Mohamed Hassan Qaed, an al-Qa'ida operative from Libya, timed for the anniversary of 11 September.

Senator Bill Nelson, a member of the Senate intelligence committee, said: "I am asking my colleagues to investigate what role al-Qa'ida or its affiliates may have played in the attack."