Terror group al-Qaeda had no direct involvement in the September 2012 attack on the US consulate in Benghazi, Libya, according to a New York Times investigation.
Some US Republicans have accused the Obama administration of failing to admit the involvement of terrorist groups.
But the New York Times (NYT) said a local Islamist militia leader was key behind the attacks.
According to the BBC, the NYT's report is based on months of interviews with local residents who have knowledge of the events of 11 September 2012 and American officials linked to a criminal investigation.
Findings suggested that it was an organised attack planned by local militias.
The NYT reported that the assault was neither 'meticulously planned', nor 'spontaneous', but was 'fuelled in large part' by anger at the video.
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The paper's investigation 'turned up no evidence that al-Qaeda or other international terrorist groups had any role in the assault'.
According to the report, the NYT said the attack was led by fighters who had benefited from Western support during the uprising against long-time Libyan leader Col Muammar Gaddafi.
It said a central figure was an eccentric, malcontent militia leader, Ahmed Abu Khattala.
Ambassador Chris Stevens was one of four Americans to die. The others killed were another state department worker and two former Navy Seals.