(CNN) -- In the most detailed explanation yet of the attack in Benghazi, Libya, that killed U.S. ambassador Christopher Stevens and three other Americans, two senior State Department officials said there was no prior indication that an attack was imminent.
The officials, who briefed reporters on background in a conference call Tuesday evening, said there was "nothing unusual" throughout the day of the attack, September 11. The ambassador held an evening meeting with a Turkish diplomat and then retired to his room in one of the compound's buildings at 9 p.m., according to the officials.
The call, a day ahead of a congressional hearing into security failings at the diplomatic mission, was an attempt to offer the State Department's view that the severity of the attack and speed at which it unfolded made it impossible to defend even with some of the increased security measures that had been requested -- but not fulfilled -- before September 11.
The first sign of a problem came at 9:40 p.m. when diplomatic security agents heard loud talking outside the compound, along with gunfire and explosions.
Asked whether the attack was a spontaneous assault taking advantage of a demonstration, as originally asserted by Obama administration officials, one senior official said, "That was not our conclusion."
"We don't necessarily have a conclusion," the State Department official said.
The two senior officials offered riveting detail of the attack, by what one official described as "dozens of armed men," marauding from building to building in the enormous complex and, later, firing mortars on a U.S. annex less than a mile away.
The attack created havoc on the compound, which had four buildings, including one that was used as a residence with bedrooms. The ambassador and two of his security personnel took refuge in a fortified room in the residence but the attackers penetrated the building, said the official. The attackers poured diesel fuel around the building and lit it on fire. The three men decided to leave the safe haven and move to a bathroom to be able to breathe.
In the chaos and smoke they were unable to find the ambassador. One official said it still is unclear how the ambassador got to the hospital where he was declared dead. Hospital personnel found his cell phone in his pocket and began calling numbers. That, the official says, is how U.S. officials learned where he was.
The officials echoed what administration officials have maintained since the attack: that U.S. and Libyan security personnel in Benghazi were outmanned and that no reasonable security presence could have fended off the assault.
As one official said: "The lethality and the number of armed people is unprecedented. There had been no attacks like that anywhere in Libya -- Tripoli, Benghazi or anywhere -- in the time that we had been there. And so it is unprecedented, in fact, it would be very, very hard to find precedent for an attack like (it) in recent diplomatic history."