Scott Wickland testifies in Ahmed Abu Khattala trial

Militants armed with AK-47s and grenades banged the butts of their guns against the locked cage that stood between them and the safe room inside the U.S. mission in Benghazi, Libya, where the three men hunkered down inside thought they were going to die.

Two of them did.

Prosecutors on Monday opened the case against Ahmed Abu Khattala, the man they say masterminded the 2012 assault against the compound, with testimony from State Department Special Agent Scott Wickland, who was in the cage with Ambassador J. Christopher Stevens and information management officer Sean Smith.

The government said that while Mr. Abu Khattala may not have set the fire that killed Stevens and Smith, he planned, encouraged and incited the attack because he “hates Americans with a vengeance.”

“Abu Khattala set the attack in order,” Assistant U.S. Attorney John Crabb said Monday as he gave an opening statement in the trial. “Abu Khattala incited others to do his dirty work.”

Four Americans died in the Sept. 11, 2012 attack — Stevens and Smith from smoke inhalation at the mission, and security officers Tyrone Woods and Glen Doherty when they were hit by mortar rounds at a nearby CIA-run annex.

Mr. Wickland testified for nearly three hours in federal court in the District of Columbia on Monday, offering a dramatic first-person account of the beginning of the attack and firefight at the compound.

On the night of the attack, he said, he was relaxing outside, feeling relieved that nothing bad had happened on Sept. 11, when he heard chanting outside the compound. Sensing it was a bad sign, he went to grab his firearms and protective gear from his room and heard the sound of gunfire and screams relayed over his radio.

He ran to get Stevens and Smith, and the men holed up inside a safe room that was caged in by a panel of locked bars when attackers breached the doors of the building.

“I didn’t think we were going to make it out of there alive, and I don’t think they did either,” Mr. Wickland said of his companions.

Through a window, Mr. Wickland said, he could see two men who had grenades on their belts banging on the metal bars trying to break through to the safe room, and he thought for sure they would use them to blow open the locks.

But the men weren’t able to get through and eventually left. By then, someone had set fire to the building and black smoke began to fill the room.

“I turned to Ambassador Stevens and Smith and said, ’We are going to go to the bathroom. We can make it,’” Mr. Wickland said.

But as the three men crawled together toward the bathroom, it became more difficult to see and breathe.

“Within that 8 meters, they disappeared,” he said.

Mr. Wickland banged on the floor hoping they might hear him, but all he heard in return was the sound of gunshots outside and the crackling of the fire inside.

“To this day, I don’t even know where they went. I was right next to them and then that’s it,” he said.

Mr. Abu Khattala sat quietly beside his attorneys during the testimony, listening to an Arabic translation of the proceedings through headphones. He wore a white, untucked dress shirt, black pants and sneakers, and had a long white beard.

In the government’s opening statement, prosecutors played snippets of security footage showing Mr. Abu Khattala walking into the Benghazi mission with an AK-47 in hand after the gates were breached, and promised testimony from witnesses about stocking up on weapons before the attack, and his claims of credit afterward.

They also said cellphone records would show he was in contact frequently with a number of associates who were at the mission during the attack.

One key witness, referred to as “Ali,” was paid $7 million by the U.S. government for providing information to investigators, Mr. Crabb said. Ali befriended Mr. Abu Khattala after the attack at the request of the U.S. government and provided details about his whereabouts and ultimately arranged the 2014 trip during which the military captured Mr. Abu Khattala.

Mr. Crabb said Ali will testify that at a meeting in Libya, Mr. Abu Khattala said he “attacked the American Embassy” and later told Ali privately that “he would have killed all the Americans that night … if others had not gotten involved and stopped me.”

Defense attorneys questioned the motivation of the witnesses involved in the case, as well as why Mr. Abu Khattala is the only man on trial for the Benghazi attack.

“There are people who are not Ahmed Abu Khattala who were the masterminds of the attack,” said Jeffrey Robinson, one of his attorneys.

Because that information is classified, the details about some of the other individuals involved in the attack and how the government learned about them will likely not be shared during the course of the trial.

“So why is Mr. Abu Khattala sitting in that chair?” Mr. Robinson said. “He is sitting there because he was easy.”

He said Mr. Abu Khattala was at the mission but did not participate. Instead, he showed up because he heard about a protest taking place there. By the time he arrived, the building where Stevens died was already on fire, the lawyer said.

Prosecutors said Mr. Abu Khattala later went to blockades to prevent anyone from providing help to the mission while it was under attack, but Mr. Robinson said he was telling people to stay clear of the area because of the gunfire.

“He was trying to keep people safe,” Mr. Robinson said. “Mr. Abu Khattala never stopped anyone from coming to the help of the American mission.”

Mr. Abu Khattala has pleaded not guilty to the 18 criminal counts, which range from murder of an officer of the United States to conspiracy to provide material support to terrorists. He faces a maximum sentences of life in prison.

A panel of 12 jurors and three alternates — six men and nine women — are hearing the case.

Legal analysts say the trial has the potential to showcase the value of prosecuting terrorism cases in American courts rather than military tribunals and to provide a measure of closure for victims’ families and the American public. But if Mr. Abu Khattala is acquitted, it could be a huge embarrassment for prosecutors.

Mr. Abu Khattala’s defense attorneys have tried unsuccessfully to challenge various aspects of his detention and interrogation — including whether his rights were violated when he was questioned by CIA and FBI officials and transported to the U.S. over a two-week period on a slow-moving Navy ship.

Statements Mr. Abu Khattala gave to FBI agents during his interview on the ship will be used in the case, though Mr. Robinson noted that the interviews were neither recorded nor videotaped and the jury will instead be told what Mr. Abu Khattala said by the agents — two of whom sat in the courtroom behind prosecutors during opening statements.