Dozens of armed men invaded the US consulate in Benghazi last month setting it on fire and hunting down staff, US officials said in a dramatic, detailed account of the deadly assault in Libya.
There had been no intelligence to warn that an attack was in the works and in the hours before the streets outside the compound had been calm, they said.
The new account contradicts initial reports by State Department officials made in the hours and days after the September 11 that it was a "spontaneous" attack which arose out of a protest against an inflammatory anti-Islam film.
"There was no actionable intelligence of any planned or imminent attack," one top State Department official said, giving a detailed description and timeline of the massive attack on the diplomatic compound and a nearby annex.
Ambassador Chris Stevens, who died in the attack, had stayed in the compound that day as it was the 11th anniversary of the September 11, 2001 attacks and had a series of meetings. He walked with his last guest, a Turkish diplomat, to the compound gates around 8:30 pm local time.
"They say goodbye, they're out in the streets. Everything is calm, at 8:30pm there's nothing unusual, there has been nothing unusual during the day at all," a second official said.
Asked why initially State Department officials had said there had been a protest against the amateur video "Innocence of Muslims," the first official said that was a question "for others. That was not our conclusion."
Speaking ahead of Wednesday's first public congressional hearing into alleged security failures at the consulate, the official said it was difficult to say what kind of security would have been needed to repel such an attack.
"The lethality and the number of armed people is unprecedented. There have been no attacks like that in Libya, Tripoli or Benghazi or elsewhere in the time that we have been there," the official told reporters.
"It would be very, very hard to find a precedent for an attack like that in recent diplomatic history."
The administration of President Barack Obama has come under fire from Republican foes for the contradictory reports which have come out about the attack, amid allegations of security failures.
On a conference call with reporters, two State Department officials said the fierce attack in eastern Benghazi erupted around 9:40 pm local time on September 11 just after Stevens had gone to bed.
Stevens and three other US diplomatic staff died when dozens of men wielding heavy arms first overran the compound, and later waged a sustained attack on an annex about two kilometers (a mile) away.
Gunfire and explosions first erupted, shattering the calm outside the compound, and agents manning security cameras saw "a large number of armed men flowing into the compound," said the first official.
There were five US diplomatic security agents at the main compound -- two who had traveled with Stevens from Tripoli as well as three posted in Benghazi -- and four members of local Libyan security, supplied by the 17th February brigade militia tasked by local Libyan authorities with ensuring security.
The armed attackers doused the outside of one of the buildings with diesel, setting it alight and then invaded the main residence, pouring fuel over furniture and starting a blaze, which let off plumes of thick, choking smoke.
An armed American security agent alerted Stevens, and together with US information manager Sean Smith they took refuge in a fortified safe haven, equipped with medical supplies and water, in a closet on the bedroom floor of the main residence.
But the three men soon found it hard to breathe and after moving to a bathroom with a grilled window, decided they had to make a break for it despite tracer bullet fire and grenades raining around the compound inside.
In their bid to escape, the three were separated. Smith died in the blaze, and his body was found during desperate search attempts by US security while Stevens somehow was taken to a hospital and his body later returned to US diplomatic staff that night.
The main compound was then evacuated and the remaining security forces fled through chaotic streets in an armored vehicle for the nearby annex, which then around 4:00 am came under mortar fire.
"It is precise and some of the mortar fire lands on the roof of the annex. It immediately kills two security agents that are there, and severely wounds one of the agents who had come from the compound," the first US official said.
Reinforcements had arrived from Tripoli and the annex was then evacuated, and all the staff, dead and wounded were flown in two flights back to Tripoli.