The US consulate in Benghazi, where the US ambassador to Libya died in an attack on Tuesday, was not given the standard security contract offered to many American diplomatic missions in the Middle East, private military contractors have told the BBC.
The consulate's walls were breached in just 15 minutes, guards were outgunned and overwhelmed and four US personnel were killed, including the Ambassador, J Christopher Stevens.
US embassies and consulates in areas of the world where they are deemed liable to attack are usually offered a formal security contract called a Worldwide Protective Services Agreement, known in the industry as a 'Wips'.
The contract, or so-called tasking order, is between the US state department and any one of several major private military contractors such as DynCorp International and Aegis Defence Services.
Under this agreement, extensive security precautions are put in place, including low-profile armoured vehicles, run-flat tyres, sufficient weapons, ammunition and trained personnel, as well as a tried and tested command and control system.
But sources have told the BBC that on the advice of a US diplomatic regional security officer, the mission in Benghazi was not given the full contract despite lobbying by private contractors.
Instead, the US consulate was guarded externally by a force of local Libyan militia, many of whom reportedly put down their weapons and fled once the mission came under concerted attack.
Inside the consulate, the defenders - consisting of a small group of Libyans and private US contractors who had formerly served in the US military including the elite Navy Seals - returned fire and put into action a fall-back plan to evacuate staff to a second building.
But the defenders were quickly outgunned by the sizable and determined attacking force that used heavy weapons including rocket-propelled grenades prompting investigators to consider whether Tuesday's attack had in fact been planned in advance by a jihadist group.
"This was a well-crafted military operation [by the attackers]," said former Libyan jihadist Noman Benotman. "They would have carried out at least two weeks of surveillance."
With fires blazing inside the compound within minutes of the attack beginning, the US ambassador became separated from other staff in thick smoke, which is believed to have caused him to suffocate.
Meanwhile the attackers appeared to know exactly where staff were being taken to and fired on a second building supposed to be a safe haven, prompting suspicions they had a prior informant inside the mission.
The investigation into the attack is being conducted jointly by the FBI, the US department of justice and the Libyan authorities, with a report due to be submitted to the US state department.
Given the unstable security situation in Benghazi and eastern Libya that has developed this year, it is surprising that security precautions for such a sensitive diplomatic mission were not more robust.
The northeast of Libya, especially around the town of Darna, has long been a home for jihadists, many of whom travelled to Iraq to fight the US military or become suicide bombers.
Both US and British diplomats in Benghazi came under attack from suspected Islamist militants in June, as did the International Committee of the Red Cross.
Britain then closed down its permanent presence there that month, moving staff back to the capital Tripoli.
This week the UK Foreign Office altered its travel advice for the region, warning against all travel to Benghazi.