The key suspect of a deadly 2012 terror attack on the US mission in the Libyan city of Benghazi that killed four Americans, including the US Ambassador to Libya, pleaded not guilty during a brief appearance before a federal court here.

Ahmed Abu Khatallah, 43, arrested by US Special Forces in a covert operation in Libya earlier this month, has been indicted by a federal grand jury on a charge of conspiracy to provide material support and resources to terrorists, knowing and intending that these would be used in preparation for and in carrying out a killing in the course of an attack on a federal facility.

If proved guilty, he could be imprisoned for life or be given the death penalty.

Four American nationals including US Ambassador to Libya J Christopher Stevens were killed on September 11, 2012 when gunmen attacked the US consulate and set it on fire.

A CIA outpost was also targeted in the attacks that have become a flashpoint in US politics.

Khatallah has been accused of being a senior member of Ansar al-Sharia, a Libyan “terrorist” group responsible for several attacks and assassinations.

Khatallah, who has a bushy grey beard and wore a dark sweatshirt and jail slippers, arrived in Washington after being captured in Libya two weeks ago and was transported via a Navy ship. He spoke very little during the court appearance yesterday.

“Now that Ahmed Abu Khatallah has arrived in the US, he will face the full weight of our justice system,” said US Attorney General Eric Holder.

“We will prove, beyond a reasonable doubt, the defendant’s alleged role in the attack that killed four brave Americans in Benghazi,” he added.

FBI Director James B Comey said: “Capturing Ahmed Abu Khatallah and bringing him to the US to face justice for his role in killing American citizens in Benghazi is a major step forward in our ongoing investigation.”

Khatallah, a Libyan national, was taken into custody earlier this month. He was initially charged in a criminal complaint that was filed under seal on July 15, 2013, and that became public on June 17, 2014.

“Ahmed Abu Khatallah’s capture and his appearance in court today were critical steps toward bringing him to justice for his role in the terrorist attacks on our diplomatic facilities in Benghazi,” said Assistant Attorney General for National Security John Carlin.