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.

Mr Stevens had been on a visit to Germany, Austria and Sweden and had just returned to Libya when the Benghazi trip took place with the US embassy's security staff deciding that the trip could be undertaken safely.

Eight Americans, some from the military, were wounded in the attack which claimed the lives of Mr Stevens, Sean Smith, an information officer, and two US Marines. All staff from Benghazi have now been moved to the capital, Tripoli, and those whose work is deemed to be non-essential may be flown out of Libya.

In the meantime a Marine Corps FAST Anti-Terrorism Reaction Team has already arrived in the country from a base in Spain and other personnel are believed to be on the way. Additional units have been put on standby to move to other states where their presence may be needed in the outbreak of anti-American fury triggered by publicity about a film which demeaned the Prophet Mohamed.

A mob of several hundred stormed the US embassy in the Yemeni capital Sanaa yesterday. Other missions which have been put on special alert include almost all those in the Middle East, as well as in Pakistan, Afghanistan, Armenia, Burundi and Zambia.

Senior officials are increasingly convinced, however, that 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 due to 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 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.

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

According to security sources the consulate had been given a "health check" in preparation for any violence connected to the 9/11 anniversary. In the event, the perimeter was breached within 15 minutes of an angry crowd starting to attack it at around 10pm on Tuesday night. There was, according to witnesses, little defence put up by the 30 or more local guards meant to protect the staff. Ali Fetori, a 59-year-old accountant who lives near by, said: "The security people just all ran away and the people in charge were the young men with guns and bombs."

Wissam Buhmeid, the commander of the Tripoli government-sanctioned Libya's Shield Brigade, effectively a police force for Benghazi, maintained that it was anger over the Mohamed video which made the guards abandon their post. "There were definitely people from the security forces who let the attack happen because they were themselves offended by the film; they would absolutely put their loyalty to the Prophet over the consulate. The deaths are all nothing compared to insulting the Prophet."

Mr Stevens, it is believed, was left in the building by the rest of the staff after they failed to find him in dense smoke caused by a blaze which had engulfed the building. He was discovered lying unconscious by local people and taken to a hospital, the Benghazi Medical Centre, where, according to a doctor, Ziad Abu Ziad, he died from smoke inhalation.

An eight-strong American rescue team was sent from Tripoli and taken by troops under Captain Fathi al- Obeidi, of the February 17 Brigade, to the secret safe house to extract around 40 US staff. The building then came under fire from heavy weapons. "I don't know how they found the place to carry out the attack. It was planned, the accuracy with which the mortars hit us was too good for any ordinary revolutionaries," said Captain Obeidi. "It began to rain down on us, about six mortars fell directly on the path to the villa."

Libyan reinforcements eventually arrived, and the attack ended. News had arrived of Mr Stevens, and his body was picked up from the hospital and taken back to Tripoli with the other dead and the survivors.

Mr Stevens' mother, Mary Commanday, spoke of her son yesterday. "He did love what he did, and he did a very good job with it. He could have done a lot of other things, but this was his passion. I have a hole in my heart," she said.

Global anger: The protests spread

Yemen

The furore across the Middle East over the controversial film about the Prophet Mohamed is now threatening to get out of control. In Sana'a, the Yemeni capital, yesterday around 5,000 demonstrators attacked the US embassy, leaving at least 15 people injured. Young protesters, shouted: "We sacrifice ourselves for you, Messenger of God," smashed windows of the security offices and burned at least five cars, witnesses said.

Egypt

Egypt's Islamist President Mohamed Morsi yesterday condemned the attack in Benghazi that killed the US ambassador. In a speech in Brussels, Mr Morsi said he had spoken to President Obama and condemned "in the clearest terms" the Tuesday attacks. Despite this, and possibly playing to a domestic audience, President Obama said yesterday that "I don't think we would consider them an ally, but we don't consider them an enemy".

Demonstrators in Cairo attacked the mission on Tuesday evening and protests have continued since.

Iraq

Militants said the anti-Islamic film "will put all the American interests Iraq in danger" and called on Muslims everywhere to "face our joint enemy", as protesters in Baghdad burned American flags yesterday. The warning from the Iranian-backed group Asaib Ahl al-Haq came as demonstrators demanded the closure of the US embassy in the capital.

Bangladesh

Islamists warned they may "besiege" the US embassy in Dhaka after security forces stopped around 1,000 protesters marching to the building. The Khelafat Andolon group called for bigger protests as demonstrators threw their fists in the air, burned the flag and chanted anti-US slogans.

Others