On the night of the attacks, armed men overran the diplomatic compound and set fire to it. Ambassador J. Christopher Stevens and another State Department employee, Sean Smith, were killed. Hours later, militants attacked the nearby C.I.A. base with mortars and small-arms fire. Two C.I.A. security contractors, Tyrone S. Woods and Glen A. Doherty, were killed, and others were wounded.
The unusual circumstances of the crime and the evidence — an orchestrated, military-style assault in a near-failed state — were a challenge for federal investigators and prosecutors. The government showed the jury large amounts of surveillance video from the attacks, but Mr. Khattala did not show up inside the diplomatic compound until the fighting was over.
Prosecutors acknowledged that there was no evidence that Mr. Khattala had personally fired any shots or set any fires, but argued that he had nevertheless helped orchestrate the attacks and aided them while they were underway. To make that case, they drew primarily on testimony from three Libyan witnesses and on a database said to be Mr. Khattala’s cellphone records, which it had obtained from Libya.
The government argued to the jury that several of the main attackers worked for Mr. Khattala in his militia — the prosecutor, Michael DiLorenzo, called them part of his “hit squad” in closing arguments last week — and pointed to phone records showing that he had been in contact with them throughout the events.
Prosecutors also presented witnesses who said that leading up to the attacks, Mr. Khattala had talked about the need to get rid of what he saw as an American spy base in Benghazi, and gathered weapons with his militia a few days before the attacks. Prosecutors said that he showed up outside the diplomatic mission before the assault there began, that he had been in phone contact with several of the attackers and that afterward he had expressed frustration that another militia leader had prevented him from killing more Americans as they were evacuated.
Defense attorneys, however, sought to raise doubts in the minds of the jurors about whether Mr. Khattala had really played the role of on-site commander that prosecutors described. They portrayed the phone logs as murky, while also stressing that those same records, if credible, suggested that Mr. Khattala had already gone home hours before the mortar attack on the C.I.A. annex.
They also questioned whether it was really possible to identify who was in the blurry surveillance video. And they portrayed the government’s Libyan witnesses as liars and con men who were probably making things up to help the government because they were Mr. Khattala’s enemies and because the United States paid them. In closing arguments, Ms. called the prosecutors’ case a “house of cards,” adding, “There are any number of reasons to doubt in this case.”
Prosecutors, however, also hedged, telling jurors that the constellation of evidence, as a whole, was sufficient to find Mr. Khattala guilty of the crimes with which he was charged, both as a conspirator in the attacks and as someone who aided and abetted them as they unfolded.
“We don’t have to prove he was a commander,” Mr. DiLorenzo said at one point. “Even if he was a lookout, that was aiding and abetting. But this tells us what he was truly doing was being the on-scene commander.”
Emotional overtones complicated the assessment of the facts. Ms. Peterson told the jurors that prosecutors were trying to manipulate them, such as by calling witnesses who dwelled on the tragic details of the attacks — though the defense did not contest that material — and by subtly invoking feelings of tribal nationalism, like referring to “our” ambassador and compound.
During the rebuttal phase of closing arguments, another prosecutor, Julieanne Himelstein, frankly embraced that strategy, delivering an passionate appeal to the jury to hold Mr. Khattala responsible for the deaths of four men she described as “our son” and an “American son.” Calling Mr. Khattala “a stone-cold terrorist,” she reminded the jury of video showing attackers stomping on an American flag while rampaging in the compound and, pointing at the defense table, declared, “How dare you!”
Supporters of using military commissions rather than civilian courts to prosecute terrorism cases seized on the verdict in the African embassy bombings as demonstrating the failure of the civilian court system. In the years since, the military commissions system has had difficulty bringing contested cases to trial, while several of its few convictions, from plea deals, were overturned on appeal.
The Obama administration refused to bring any new detainees to the wartime prison at Guantánamo Bay, Cuba, where the military commissions system is operating. But a trial before the war crimes tribunal was probably not an option for Mr. Khattala. That system has jurisdiction only over members of groups with which the United States is at war, such as Al Qaeda, and while Mr. Khattala shared an Islamist ideology, scant evidence existed that he was a member of any such faction.
His case also highlighted the role of military commandos who had the unusual task of handling an informant in Libya for the F.B.I.
The military’s Joint Special Operations Command began working with the informant, a Libyan businessman, at the end of 2012. Eventually, F.B.I. agents in New York and federal prosecutors believed they had gathered enough evidence to prosecute Mr. Khattala, including many incriminating statements that the informant had overheard.
In June 2014, the informant lured Mr. Khattala to a villa on the Libyan coast near Benghazi, where an F.B.I. agent and elite Delta Force commandos were waiting in ambush. Mr. Khattala was taken to an American warship and interrogated for days. His statements helped investigators identify another suspect in the Benghazi attacks, Mustafa al-Imam, who was captured in late October in Libya. Mr. Imam has pleaded not guilty.
The 40-year-old informant is a college-educated businessman who once studied in Canada. He approached the United States and offered his help because he thought extremists were tearing apart Benghazi. He testified under a pseudonym.
The informant told jurors that many of Mr. Khattala’s associates were outside the diplomatic compound on the night of the attack. The associates and Mr. Khattala, who was armed with an AK-47 assault rifle, were captured on surveillance video.
The informant was paid $5,000 a month as he collected information on Mr. Khattala and later earned a $7 million reward. The F.B.I. also made arrangements to move the informant and his family to the United States and pay their living expenses.
While the defense team sought to portray the informant as financially motived, he testified that he decided to work for the United States because he was opposed to extremists like Mr. Khattala, whom he called a murderer.