Story highlights The attack began after protesters began massing outside the consulate

A small group of militants opened fire on the complex and gained entry

The militants set fire to the main building with the U.S. ambassador still inside

Control was regained more than four hours later

Inside a safe room of the U.S. consulate in Benghazi, Libya, Ambassador Chris Stevens took cover with two other foreign service officers.

Just minutes earlier, a group of heavily armed militants launched an attack on the complex, pelting it with gunfire and rocket-propelled grenades.

The attackers ignited a fire in the building and the safe room was filling with smoke.

Stevens, Foreign Service information management officer Sean Smith and a U.S. regional security officer would soon be forced to leave their temporary refuge from the assault.

Senior administration officials called the conditions inside "awful," describing "incredibly thick smoke and fire."

The officials, while still trying to piece together the details of how the Tuesday night consulate attack unfolded, began Wednesday to give a clearer picture of events.

The violence began around 10 p.m. Tuesday amid a protest by the radical Islamist group Ansar Al-Sharia against a film mocking Islam's prophet.

Photos: Photos: Attack on U.S. mission in Benghazi Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Attackers set the U.S. mission in Benghazi, Libya, on fire on September 11, 2012. The U.S. ambassador to Libya, Christopher Stevens, and three other U.S. nationals were killed during the attack. The Obama administration initially thought the attack was carried out by an angry mob responding to a video, made in the United States, that mocked Islam and the Prophet Mohammed. But the storming of the mission was later determined to have been a terrorist attack. Hide Caption 1 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Obama and Clinton stand at Andrews Air Force Base as the bodies of the four Americans killed are returned on September 14. Hide Caption 2 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A desk sits inside the burnt U.S. mission on September 13, two days after the attack. Hide Caption 3 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Damage is seen inside the U.S. mission on September 13. Hide Caption 4 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A lounge chair and umbrella float in the swimming pool of the U.S. mission on September 13. Hide Caption 5 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Demonstrators gather in Libya on September 12 to condemn the killers and voice support for the victims. Hide Caption 6 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – U.S. President Barack Obama, with Secretary of State Hillary Clinton on September 12, makes a statement at the White House about Stevens' death. Hide Caption 7 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A burnt vehicle is seen at the U.S. mission in Benghazi on September 12. Hide Caption 8 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – People inspect the damage on September 12. Hide Caption 9 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A small American flag is seen in the rubble on September 12. Hide Caption 10 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A man stands in part of a burned-out building of the U.S. mission on September 12. Hide Caption 11 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Smoke and fire damage is evident inside a building on September 12. Hide Caption 12 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Half-burnt debris and ash cover the floor of one of the U.S. mission buildings on September 12. Hide Caption 13 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – The U.S. mission is seen in flames on September 11, the day of the attack. Hide Caption 14 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A protester reacts as the U.S. mission burns on September 11. Hide Caption 15 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle and the surrounding area are engulfed in flames on September 11. Hide Caption 16 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Flames erupt outside of a building on September 11. Hide Caption 17 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle burns during the attack on the U.S. mission on September 11. Hide Caption 18 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Onlookers record the damage from the attack on September 11. Hide Caption 19 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – Onlookers walk past a burning truck and building on September 11. Hide Caption 20 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – A vehicle sits smoldering in flames on September 11. Hide Caption 21 of 22 Photos: Photos: Attack on U.S. mission in Benghazi Attack on U.S. mission in Benghazi – People duck flames outside a building on September 11. Hide Caption 22 of 22

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Four hours later, the consulate was destroyed , its walls blackened by shooting flames. Four Americans, including Stevens and Smith, were dead.

Within 15 minutes of the first gunshots, the attackers were able to enter the main compound of the consulate complex and set it ablaze.

Stevens, Smith and a regional security officer were the only people inside the building at the time.

Photos taken Tuesday night show flames shooting from every orifice of the building as militants wielding weapons walk through the compound.

The heavy, black smoke added to the confusion of the situation and the three diplomatic officers became separated as they evacuated the safe room, the administration officials said.

Meanwhile, American and Libyan security personnel, forced to fend off an attack on the outside and the raging fire within, were grappling with how to handle the increasingly complicated scene, U.S. officials said.

The regional security officer with Stevens and Smith managed to escape the building and returned with others to try to rescue the men. For Smith, the rescue came too late. He had already succumbed to smoke inhalation by the time they arrived. Stevens, in Benghazi on a short trip from the capital Tripoli, was missing, apparently able to make his way out unnoticed in the chaos.

About 45 minutes after the first gunshots rang out, security personnel attempted to retake the main compound, but were repelled by heavy gunfire and retreated to an annex in the complex.

They made a second attempt at 11:20 p.m. -- more than an hour after the attack began -- and successfully regained control of the main building.

But the incident was far from over. The gunbattle continued and the attack moved from the main compound to the complex's annex.

"It was during that time that two additional U.S. personnel were killed and two others were wounded," one of the senior administration officials said.

Finally, around 2:30 a.m., four and a half hours into the attack, U.S. security personnel, assisted by Libyan forces, took back control of the entire consulate complex.

Still unknown at that time, however, was Stevens' whereabouts.

Libyans said bystanders had helped carry an unconscious Stevens to a hospital. However, U.S. officials could not confirm that account.

"We are not clear on the circumstances between the time he got separated from the rest of the group inside the burning building to the time we were notified that he was in a Benghazi hospital," the senior administration official said. "We were not able to see him until his body was returned to us at the airport."

Also unclear Wednesday was the significance of the timing of the attack, which fell on the 11th anniversary of the September 11 attacks on New York and Washington and coincided with a violent protest at the U.S. Embassy in Cairo, Egypt.

State Department officials said the two incidents at the diplomatic missions were not related and said they believe the Benghazi violence was a "clearly planned attack."

"It was not an innocent mob," one senior official said. "The video or 9/11 made a handy excuse and could be fortuitous from their perspective but this was a clearly planned military-type attack."

U.S. President Barack Obama, however, said it was too early to say whether the attack was pre-planned.

As dawn broke Wednesday, the magnitude of the previous night's events were felt by both Americans and Libyans. Charred debris and ash littered the floor of the burned-out consulate. A small American flag lay amid the rubble. And both the governments of the United States and Libya, on this "very, very difficult day," mourned the loss of Stevens, who had helped save Benghazi during last year's revolution.

"He risked his life to stop a tyrant, then gave his life trying to help build a better Libya," U.S. Secretary of State Hillary Clinton said.