Fire damage shown on Oct. 3, 2012, in the main compound villa in which Ambassador J. Christopher Stevens suffered the asphyxiation that killed him at the U.S. mission in Benghazi. (Michael Birnbaum/The Washington Post)

State Department security officers described scenes of chaos and death as September 2012 assaults unfolded at the U.S. diplomatic mission and nearby CIA post in Benghazi, Libya, as the federal trial continued Tuesday of the accused ringleader of the attacks, Ahmed Abu Khattala.

In six hours of testimony, Diplomatic Security special agent Scott Wickland stood as one of the sole surviving human links between the two attack sites and several victims.

As bodyguard to U.S. Ambassador J. Christopher Stevens, Wickland was the last person to see him and an aide alive as they tried to escape a burning mission villa. Wickland also was at the wheel of an armored car evacuating U.S. security personnel that ran a gauntlet of hostile fire to the CIA annex, only to have the attack trail them and mortar rounds kill two more Americans and wound one of Wickland’s close friends just feet away from him on a rooftop.

"They all looked like they had seen a ghost," said Wickland, describing agents' reactions at finding him alive outside the villa. "My face was covered in soot, my eyes were black, my teeth were black."

Wickland's riveting on-the-ground testimony in a federal courtroom in Washington set the scene of Americans under fire for prosecutors, but by itself did not directly implicate Abu Khattala, a Libyan militia leader captured overseas by U.S. commandos in 2014.

U.S. Ambassador J. Christopher Stevens in the lobby of the Tibesty Hotel in Benghazi, Libya, in an April 11, 2011, photo. (Ben Curtis/AP)

Abu Khattala has pleaded not guilty to 18 counts including murder, attempted murder, material support of terrorism and destruction of U.S. facilities in the Benghazi attacks of Sept. 11 and Sept. 12, 2012. He faces a life sentence if convicted.

Stevens was the first U.S. ambassador killed since 1979 in violent actions.

Wickland described how first he heard screams on a security radio as Libyan guards were shot and killed outside the diplomatic compound, then more shots as close to 80 militants overran the grounds and triggered explosions outside Stevens's quarters after 9:45 p.m. Sept. 11, 2012.

Hunkered down in a closet converted into a safe room protected by metal bars in a building designated Villa C, Wickland said he guarded Stevens, handed his shotgun to communications aide Sean Patrick Smith, and then felt the pressure change in his ears as what he thought was a rocket-propelled grenade blew open the outer doors of the building.

"I didn't know that we were going to make it out of there alive," said Wickland, a former U.S. Navy combat rescue swimmer who had arrived in Benghazi only six weeks earlier.

A man carries a large poster of a map of Benghazi, Libya, into federal court in Washington on Monday, as a trial opened for Ahmed Abu Khattala, the suspected mastermind of the 2012 attacks on two U.S. compounds in Benghazi, Libya, that killed four Americans. (Andrew Harnik/AP)

The attackers retreated, but the villa was set afire, pumping acrid, black smoke from ceiling to floor.

With one hand on his M-4 rifle and the other on Stevens, Wickland said he last saw the ambassador and Smith crawling behind him as they tried to reach a bedroom with a window. 'To this day, I don't even know where they went," Wickland said, his voice catching with emotion during testimony. "I was right next to him and then, that's it."

Wickland climbed out of the window alone, then reentered the compound four times to try to find the men without success, vomiting from the smoke, coming under the fire when he was outside, and calling on his radio for help , he said.

Finally, he climbed to the roof, pulling up a ladder after him, testifying, "That's when I knew [Stevens and Smith] were dead."

Wickland was reached by other agents, still barefoot because the attack began so suddenly he did not have time to put on shoes. A Tripoli-based U.S. agent found Smith's body and carried it out with Diplomatic Security agent Dave Ubben.

Reluctant to abandon Stevens but believing him dead and fearing a renewed attack, two carloads of U.S. security personnel fell back to the CIA annex, little more than a kilometer away. Wickland drove one car, and as he passed a crowd of about 90 armed Libyans, they opened fire , striking "the car hundreds of times."

Once inside the gate, Global Response Staff CIA contractors were "shocked" to see the damage, he said. An explosion that Wickland suspected was tossed gelatin dynamite "ripped off" much of the rear of the vehicle, two tires were shot out, windows were spiderwebbed and the body appeared pocked by hundreds of bulletmarks.

Wickland went into a building, while Ubben and the GRS team in the other vehicle, including Tyrone S. Woods, a 41-year-old former Navy SEAL, took positions on the rooftop. Another GRS team from Tripoli arrived, and one member, Glen Doherty, 42, a former SEAL and medic, checked on Wickland, who had been struggling to breathe.

"I was pretty grateful for that. He seemed just like a good guy . . . and then right away, he just went up and joined my friends on the roof," Wickland said. "Right after that I hear the explosions happen."

Four suspected mortar rounds hit, he said. Doherty and Woods were killed. GRS member Mark "Oz" Geist, then 46 and a former police chief, suffered wounds to his neck, arm and abdomen; Ubben, 36, who had come to Wickland's rescue, had severe wounds to his face, arm and leg.

With casualties mounting, the Americans evacuated through Benghazi’s airport, where Stevens’s body was returned by friendly Libyans.

Wickland said he left Libya having never fired a shot through the attacks, and has not gone back.