(NBC News) -- In addition to apportioning blame for the Sept. 11 attack on the U.S. diplomatic mission in Benghazi, Libya, the independent report released Tuesday provides a gripping and authoritative account of the assault, detailing the hours leading up to the deaths of four Americans -- including Ambassador Chris Stevens -- and the heroic efforts of American security agents to save the besieged envoy and his support staff.
The report by the independent review panel, commissioned by U.S. Secretary of State Hillary Clinton, states that while there was no specific threat to the diplomatic post known as "Special Mission Benghazi" timed to the 11th anniversary of the Sept. 11 terrorist attacks, Stevens and his staff were well aware that the city was essentially a lawless zone that had seen an increasing number of security incidents in the preceding months.
Stevens, 52, who was familiar with Benghazi after serving as special envoy to the National Transitional Council, which led the uprising that eventually toppled Libyan strongman Moammar Gadhafi, in 2011, returned to the city from Tripoli for the first time since being appointed ambassador on Sept. 10, accompanied by two security agents on the trip, the report said.
Stevens had made his plans to visit Benghazi independently of Washington, D.C., according to the report: "Plans for the Ambassador's trip provided for minimal close protection security support and were not shared thoroughly with the Embassy's country team, who were not fully aware of planned movements off compound."
Stevens, whose visit was timed to fill staffing taps at the mission, also wanted to reconnect with local contacts from his time as special envoy, it said. He planned to stay through Sept. 14.
There were eight Americans, including Stevens, at Special Mission Benghazi at the time, though that number was reduced to seven when a staff member who had completed his assignment to the facility left on the morning of Sept. 11. Also guarding the 8-acre compound were three armed members of the February 17 Martyr's Brigade, an umbrella group of local militias. There were supposed to be four militia guards on duty that night, but one was absent because of an illness in the family, the report said.
On the day of his arrival, an American guard gave Stevens a tour and pointed out an escape hatch in his bedroom. Later in the day, the ambassador had dinner with Benghazi city council members.
The next morning, Sept. 11, Stevens decided to stay on the compound, mindful that it was the anniversary of the 9/11 attacks.
At 7:40 p.m., Stevens and an American guard walked a Turkish diplomat to the compound's main gate. Everything appeared normal.
At 9 p.m., Stevens and Sean Smith, who had arrived in Benghazi the week before to provide communications support, retired to their quarters for the night. Three of the guards sat together outside, behind the ambassador's quarters, as the fourth guard watched a video in the common area and the fifth guard stayed in the office.
At 9:42 p.m., shots rang out and an explosion was heard. Looking at security cameras, the guard in the office saw that dozens of individuals were entering the compound through the main entrance, some chanting. The guard immediately hit the alarm and yelled a warning over the radio. He alerted the ambassador by cell phone.
Outside, the February 17 militia guards were fleeing, the report said.
The guard who had given Stevens the tour headed to their quarters, outfitted him and Smith with body armor and led them to the safe area he had pointed out earlier in the day. According to the report, the guard was armed with an M4 rifle, a shotgun and a pistol. He gave Stevens his cell phone. Stevens called his local contacts and the U.S. Embassy in Tripoli. As he told his colleagues that the compound was under attack, the call was cut off.
Elsewhere, guards hunkered down, barricading themselves in rooms.
Between 9:45 and 10 p.m. the crowd outside set fire to cars parked near the compound.
Then they headed toward the ambassador's quarters.
The militants broke into the quarters, known as Villa C, and started tearing apart the living room, the report said. When they arrived at the safe area gate, they banged on it, according to the report.
Stevens, Smith and the guard remained quiet so as not to let the militants know they were there. Looking out, the guard recalled that it looked like the lights were dimming. Rather, it was smoke that was obscuring his vision, as the invaders had set fire to the building.
As smoke engulfed Villa C, the guard led Stevens and Smith to a bathroom with a window. The guard opened the window and stuffed towels under the door, but "thick black smoke made breathing difficult and reduced visibility to zero," the report said.
The guard tried to lead Stevens and Smith to another room with a window, crawling on his hands and knees while yelling and banging on the floor to guide them. But when he reached the other room, opened the window grill and fell outside onto a patio, he realized Stevens and Smith hadn't followed him, according to the report.
The guard climbed back inside several times but couldn't find the two men. He then climbed a ladder to the roof, broke a skylight hoping to ventilate the building and radioed for help.
The militants had backed off by this point, and the guard in the office had been able to contact the Embassy in Tripoli, the State Department in Washington, D.C., and the Annex, another building on the compound where other diplomatic officials were staying.
Other guards in the mission drove an armored vehicle to Villa C, where they found the first guard on the roof, "vomiting from severe smoke inhalation and losing consciousness," the report said.
They climbed into the building, braving the dense, suffocating smoke to search for the missing men. Reinforced by additional personnel from the Annex, a separate U.S. facility to the south of the Special Mission compound, they eventually found Smith, who was dead from apparent smoke inhalation. They couldn't find Stevens.
Without Stevens, the guards climbed back into the armored vehicle and left the compound as shots were fired at them. As they left, a man motioned them to turn into a neighboring compound.
The agents "suspected a trap, ignored this signal and continued past," according to the report. They were correct: Along their route, a group "opened fire at the vehicle's side, shattering and almost penetrating the armored glass and blowing out two tires," it said. A short time later, the armored vehicle arrived at the Annex.
By then, the U.S. Military's Africa Command had dispatched a surveillance drone to the compound. The Embassy in Tripoli chartered a private plane and deployed a seven-person security team to Benghazi.
Shortly before midnight, the Annex also came under attack from gunfire and rocket-propelled grenades. which continued for about an hour, the report said. Annex security personnel returned fire.
At 2 a.m., the Embassy in Tripoli received a call from the cell phone the guard had given to Stevens.
An Arabic-speaking man said he was with a man at the hospital with a foreigner. He described a man who looked like Stevens.
Embassy officials worried it was a trap to lure in Americans and sent a Libyan contact who confirmed that the man was Stevens, and that he had died.
Hospital staff relayed later that six Libyans, who they described as "good Samaritans" among the looters who descended on the U.S. diplomatic compound after the attack had brought Stevens' body to the hospital. They had discovered Stevens' in Villa C, pulled him out of the bedroom window and took him to the hospital, the report said. Doctors spent 45 minutes unsuccessfully attempting to resuscitate him.
At 5 a.m., the seven-man security team dispatched from the U.S. Embassy in Tripoli arrived at the Annex to reinforce the security team there. Within 15 minutes of their arrival, according to the report, militants launched five mortar rounds on the Annex in less than 90 seconds.
Three rounds hit the roof, killing security officers Tyrone Woods and Glen Doherty and severely injuring two other security officers, it said.
At approximately 6:30 a.m., all U.S. government personnel evacuated the Annex with support from a quasi-governmental Libyan militia. They arrived at the airport without incident, the report said.
As they awaited a flight to take them to Tripoli, personnel from the mission worked with Libyan contacts to have Stevens' body brought to the airport. It arrived in what appeared to be a local ambulance. A guard from the Special Mission confirmed the ambassador's identity.
Evacuees, including all wounded personnel, departed Benghazi on the chartered jet at approximately 7:30 a.m. Staff from the U.S. Embassy in Tripoli, including the Embassy nurse, met the flight at the airport there. "Wounded personnel were transferred to a local hospital, in exemplary coordination that helped save the lives of two severely injured Americans," the report said.