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. mission compound in Benghazi and a nearby U.S. annex came as protesters outside the mission 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 mission 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 mission 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 mission in Benghazi.

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

Eric Nordstrom, the regional security officer in Libya until July, had conveyed concerns about the Libyan government's ability overall to protect American diplomatic facilities.

Moreover, he sent two cables to State Department headquarters in March and again in July requesting additional security agents for the Benghazi post, but did not receive any response, according to a summary of his interview with House Oversight and Government Reform Committee Democrats that was obtained by CNN.

Nordstrom told the committee that Deputy Assistant Secretary Charlene Lamb wanted to keep the number of U.S. security personnel in Benghazi "artificially low." Nordstrom said she generally believed that extra security was unnecessary because there was a residential safe haven to fall back on in an emergency, according to the summary.

She thought the "best course of action was to assign three agents" to the Benghazi post, the summary quoted Nordstrom as saying.

Nordstrom said the facility usually had three or four agents. But he also told the committee that Undersecretary of State for Management Patrick Kennedy issued a memo in December requiring five agents be assigned to Benghazi.

Senior State Department officials have said security personnel in Benghazi were outmanned and outgunned during the attack and that no reasonable security presence could have successfully fended it off.

"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 (this) in recent diplomatic history," a senior agency official said.

Who is behind the attack?

A pro-al Qaeda group responsible for a previous armed assault on the U.S. mission in Benghazi is the chief suspect in the attack, sources tracking militant Islamist groups in eastern Libya say.

They also note that the attack immediately followed a call from al Qaeda leader Ayman al-Zawahiri for revenge for the death in June of Abu Yahya al-Libi, a senior Libyan member of the terror group.

The group suspected to be behind the assault, the Imprisoned Omar Abdul Rahman Brigades, first surfaced in May when it claimed responsibility for an attack on the International Red Cross office in Benghazi. The next month the group claimed responsibility for detonating an explosive device outside the U.S. mission and later released a video of that attack.

Noman Benotman, once a leading member of the Libyan Islamic Fighting Group and now based at the Quilliam Foundation in London, said, "An attack like this would likely have required preparation. This would not seem to be merely a protest which escalated."

Benotman, who had earlier warned of the likelihood of renewed attacks against U.S. interests in Libya, said on Tuesday that the Omar Abdul Rahman Brigades is a prime suspect in the Benghazi attack. He said he believes it is likely the deadly attack was also linked to a video statement released by al-Zawahiri on the 11th anniversary of the 9/11 attacks. In the video, al-Zawahiri confirmed the death of al-Libi, a prominent member of the al Qaeda-linked group, adding: "His blood is calling, urging and inciting you to fight and kill the crusaders."

No names have been released for those responsible for carrying out the attack.

However, a series of arrests was made in the wake of the attack, and after President Obama said he would not rest until those responsible were brought to justice.

Mohamed al-Magariaf, the head of Libya's General National Congress, said around 50 people were arrested, though another senior government official said the number was not that high.

The official said as many as 50 people have been brought in for questioning, but not all of them were arrested. They were people who were at a protest outside the mission, but there was no indication yet that they took part in the violence, he said.

Those arrests were confirmed by Mustafa Abushagur, the prime minister of Libya, who added that other people may have been involved and the investigation is ongoing.

Two Tunisians were also detained for questioning in Turkey, and U.S. Secretary of Defense Leona Panetta said there was a connection between the arrests and the attack.

"We know there is some connection, but, frankly, we really don't have all the specifics," Panetta said.

The Tunisians, who had been on a watch list provided by the United States to Turkish authorities, were being questioned at the request of the United States, a U.S. official with knowledge of the investigation said.