A grand jury has brought new charges against the suspected ringleader of the September 2012 raid on a US diplomatic post in the Libyan city of Benghazi.
Ahmed Abu Khattala, 43, faces 17 new charges, some of which may be punishable by death if convicted.
The new charges include four counts of killing a person in the course of an attack on a federal facility.
US Ambassador Chris Stevens and three others were killed in the attack, which became a political lightning rod.
"We will never relent in pursuing justice against those who commit heinous acts of terrorism against the United States," Attorney General Eric Holder wrote in a statement on Tuesday.
"Those who would do harm to our citizens - no matter how far away - should understand that our nation's memory is long and our reach is far."
Ahmed Abu Khattala
•Native of Benghazi in eastern Libya
•Construction worker by trade
•Spent several years in Col Muammar Gaddafi's notorious Abu Salim prison in Tripoli
•Formed his own small militia during the anti-Gaddafi uprising
•Denies any links to al-Qaeda but has expressed admiration for it
•Also denies any role in the attack on the US embassy in 2012, but eyewitnesses report him being there
•US state department says he is a senior leader in Islamist group Ansar al-Sharia
Other new charges levelled against Mr Khattala include one count of murder of an internationally protected person and one count of providing material support and resources to terrorists resulting in death.
Mr Khattala was taken into custody in a secret US military raid in Libya on 15 June. He was subsequently indicted on 26 June in connection with the attack on the US facility.
On 11 September 2012, gunmen stormed the US consulate in Benghazi and set it on fire.
In addition to Mr Stevens, information technology specialist Sean Smith and security workers and ex-Navy Seals Tyrone Woods and Glen Doherty were killed.
The White House initially said the attack stemmed from anti-American protests over a crude video produced in the US that was deemed insulting to Islam.
Government investigators soon determined it was an organised attack planned by local militias.
In subsequent years, Republicans have accused Mr Obama's administration of compromising security, a failure of intelligence and covering up the involvement of militant groups in order to assist the president's re-election campaign.