The United States has blacklisted two Libyan armed groups, accusing them of leading a 2012 attack on the US mission in Benghazi in which the ambassador and three other Americans died.
The Friday announcement was the first time the US officially named groups thought responsible, but the State Department insisted that did not mean they were the only ones responsible, or that there was new information.
Two Ansar al-Sharia groups set up separately in Benghazi and Derna in 2011 after the fall of Muammar Gadaffi were both designated foreign terrorist organisations.
The State Department said they "have been involved in terrorist attacks against civilian targets, frequent assassinations, and attempted assassinations of security officials and political actors in eastern Libya, and the September 11, 2012 attacks against the US special mission and annex in Benghazi.
"Members of both organisations continue to pose a threat to US interests in Libya," it added, in a statement.
Ahmed Abu Khattalah, a senior leader of Ansar al-Sharia in Benghazi, and Sufian bin Qumu, a former Guantanamo Bay detainee and the leader of the Derna group, were also both designated "global terrorists".
|Ambassador Chris Stevens
Ambassador Chris Stevens and three other US staff were killed when armed fighters overran the mission in Benghazi, and then attacked a nearby CIA annex.
So far no charges have been brought and no-one has been arrested in an investigation being led by the FBI, but a $10 million reward is being offered for any information leading to the arrest of the attackers.
"Today's announcement is not an assertion that Ansar al-Sharia in Benghazi and Ansar al-Sharia in Derna were the only two organisations whose members were involved in the attack in Benghazi," said the State Department spokeswoman, Jen Psaki.
Qumu was reportedly transferred from the US military jail in Guantanamo Bay, in 2007 to Libya and later released from prison.
The US earlier on Friday said it was organising a programme to train thousands of Libyan soldiers later this year to boost security in the country.