Ambassador Chris Stevens and three other Americans died in the attack
Al-Qaeda had no direct involvement in the September 2012 attack on the US consulate in the Libyan city of Benghazi, according to a New York Times investigation.
The US ambassador to Libya, Chris Stevens, was killed when gunmen stormed the compound and set it on fire.
Some US Republicans accuse the Obama administration of failing to admit the involvement of terrorist groups.
But the New York Times (NYT) says a local Islamist militia leader was key.
The paper bases its report on months of interviews with local residents who have extensive knowledge of the events of 11 September 2012 and American officials linked to a criminal investigation.
Initially, Washington said the attack grew out of violent protests against an anti-Islam video produced in the US.
Later findings suggested that it was an organised attack planned by local militias.
Some Republicans accused al-Qaeda of launching the assault to mark the 11th anniversary of the 9/11 attacks in the US.
‘Murkier’The NYT reports that the reality was “murkier”. The assault was neither “meticulously planned”, nor “spontaneous”, though “fuelled in large part” by anger at the video.
The paper’s investigation “turned up no evidence that al-Qaeda or other international terrorist groups had any role in the assault”.
In the aftermath of the attack, Republicans repeatedly criticised the Obama administration for blaming the video protest instead of a deliberate terrorist attack.
An investigation commissioned by the US state department found in December 2012 that security at the consulate had been inadequate but that there had been “no immediate, specific” intelligence pointing to threats.
The NYT says the attack was led by fighters who had benefited from Western support during the uprising against long-time Libyan leader Col Muammar Gaddafi.
It says a central figure was “an eccentric, malcontent militia leader, Ahmed Abu Khattala”.
Mr Khattala denies involvement, but witnesses describe him “strolling calmly through the chaos” at the compound, according to the NYT.
He is also alleged to have directed local fighters in the attack. Mr Khattala, whose exact whereabouts are unknown, was charged by US investigators in August.
Charges have also been filed against an unknown number of other alleged attackers.
Ambassador Chris Stevens was one of four Americans to die. The others killed were another state department worker and two former Navy Seals.