Federal prosecutors have opened their case against Ahmed Abu Khatallah by telling jurors he orchestrated the 2012 attack on a US diplomatic compound in Benghazi, Libya, that killed Christopher Stevens, the US ambassador, and three other Americans.
Khatallah has been awaiting trial since 2014 when he was captured by a team of US military and FBI officials in Libya and transported on a 13-day journey to the US aboard a navy vessel.
Before his capture, he was part of a revolutionary armed group aimed at overthrowing Muammar Gaddafi's government.
Republicans repeatedly accused Clinton of failing to adequately protect the diplomatic compound [Jonathan Ernst/Reuters]
The charges against Khatallah include murder and providing material support to "terrorists".
Khattala, who has a long, grey beard, entered the federal courtroom in Washington, DC on Monday and shook hands with members of his legal team.
Before the opening statements, US District Judge Christopher Cooper gave the jury 30 minutes of instructions.
"I am here to run a fair and efficient trial and to make sure the trains run on time," Cooper told them.
"You and only you are the ultimate deciders of fact in this case."
Khatallah, who wore a white shirt, monitored his trial through an interpreter. At times, he swivelled in his chair at the defence table.
In his opening statement, federal prosecutor John Crabb said Khatallah hated the US "with a vengeance" and played a leading role in organising the September 11, 2012, attack on the US diplomatic mission in Benghazi.
Khatallah "didn't do the killing by himself", he said. "He didn't light the fires, and he didn't fire the mortars but you will hear he is just as guilty as the men who lit those fires and the men who fired those mortars."
Stevens was killed in the attack in Benghazi along with three other Americans [Getty Images]
Crabb told the jury that Khatallah's "hatred simmered until it boiled over" and he organised the attack.
Stevens was killed in the attack along with Sean Patrick Smith, a state department information management officer.
Nearly eight hours later, at a CIA complex nearby, two more Americans, contract security officers, Tyrone Woods and Glen Doherty, died in a mortar attack.
The Benghazi attack stirred up a political storm in the US, where Republicans repeatedly accused Hillary Clinton, the then-secretary of state, of failing to adequately protect the diplomatic compound.
That debate stretched for years and continued throughout the 2016 presidential election during which Clinton ran unsuccessfully for president against Donald Trump.
Monday's trial represents a major test in the use of a federal court to try a foreign "terrorism" suspect, as opposed to holding him at Guantanamo Bay in Cuba where he would face military legal proceedings.
While Khatallah was being transported by navy ship from Libya to the US, he was questioned by US intelligence officials before he was read his Miranda rights and questioned by FBI agents.
He waived his rights to remain silent and have a lawyer present before speaking with the agents.
His attorneys sought to suppress the statements, saying the government had violated his rights.
However, Judge Cooper issued a ruling in August that found his statements could be admitted at trial.
Crabb alluded to prior statements by Khatallah, telling the jury the defendant had said: "I didn't do all of that by myself."