Some publicly known details of the September 11 killings of four Americans in Benghazi, Libya, including U.S. Ambassador to Libya Chris Stevens, have changed in the weeks since the attack.

U.S. officials initially said the attack on the U.S. Consulate in Benghazi and a nearby U.S. annex came as protesters outside the consulate rallied against an online video that unflatteringly portrays Islam's Prophet Mohammed. That explanation seems to have shifted as investigations progressed.

The following is the latest information that CNN has gleaned about the attack, and some unanswered questions.

Was the attack spontaneous?

Gunmen attacked the consulate around 9:40 p.m., after Stevens retired to his room at the complex following an evening meeting with a Turkish diplomat, two senior State Department officials told reporters this week.

Dead after the gun attack and fire at the complex were Stevens and State Department computer expert Sean Smith, who officials said died of smoke inhalation. The two others -- security contractors and former U.S. Navy SEALs Glen Doherty and Tyrone Woods -- died of wounds they suffered in an attack on a nearby annex.

U.S. officials initially said gunmen began attacking the complex during a protest against the inflammatory online video, after a similar protest outside the U.S. Embassy in Cairo earlier in the day. U.S. sources said it appeared the attackers used the Benghazi protest as a diversion to launch the attack.

But on September 28, Shawn Turner, spokesman for Director of National Intelligence James Clapper, said the latest information indicated the attack was wasn't spontaneous, but rather deliberate and organized, perpetrated by "extremists."

"In the immediate aftermath, there was information that led us to assess that the attack began spontaneously following protests earlier that day at our embassy in Cairo," Turner said. "As we learned more about the attack, we revised our initial assessment to reflect new information indicating that it was a deliberate and organized terrorist attack carried out by extremists."

On Tuesday, asked whether the attack was a spontaneous assault taking advantage of a demonstration, as originally asserted by Obama administration officials, a senior State Department official said: "That was not our conclusion. I'm not saying that we had a conclusion."

How did the attack unfold?

On Tuesday, two senior State Department officials emphasized that there was no prior indication such an assault was imminent, and said there was nothing unusual that day in the hours before the attack.

The first sign of a problem came 40 minutes after Stevens retired to his room, the officials said. Diplomatic security agents heard loud talking outside the compound, along with gunfire and explosions.

One of the officials said "dozens of armed men" marauded from building to building in the complex and later fired mortars on the nearby U.S. annex.

At the compound, which had four buildings, Stevens and two of his security personnel took refuge in a fortified room that the attackers were able to penetrate, one official said.

The attackers doused the building with diesel fuel and set it ablaze, and the three men decided to leave the safe haven and move to a bathroom to be able to breathe, according to the official. Stevens became separated from the security personnel in the chaos and smoke, and eventually turned up at a Benghazi hospital, where he was declared dead.

Hospital personnel found his cell phone in his pocket and began calling numbers, which is how U.S. officials learned where he was, the State Department officials said.

The complex had an armed security force of about nine people when the attack began, according to sources familiar with the incident. But that force was outmanned by the attackers, and no reasonable security presence could have fended off the assault, the two State Department officials said Tuesday.

Doherty, Woods and other security personnel were at the annex when the attack on the main compound began. The team from the annex went to the compound and rounded up consulate staff. The team also recovered the body of Smith, who died of smoke inhalation at the complex.

After the team went back to the annex, the annex came under attack, with rocket-propelled grenades and mortars among the heavy firepower used, officials told CNN.

"It was during that (second attack) that two additional U.S. personnel were killed and two others were wounded," a senior administration official said last month. Those two victims were later confirmed to be Doherty and Woods.

Were proper security measures in place?

A source familiar with Stevens' thinking told CNN that in the months leading up to his death, the ambassador worried about what he called the security threats in Benghazi and a rise in Islamic extremism.

The timing of those concerns, first reported by CNN, coincided with a request by the State Department's top security official in Libya asking for extra security for the consulate in Benghazi.

The official received no response from superiors, according to documents obtained by CNN.