WASHINGTON - The United States on Friday blacklisted two Libyan militant groups accusing them of leading a 2012 attack on the US mission in Benghazi in which the ambassador and three other Americans died.
It was the first time the State Department had officially named the groups thought to be behind the deadly assault, but a spokeswoman insisted that did not mean they were the only ones responsible, or that there was any new information about the operation.
Two Ansar al-Sharia groups set up separately in Benghazi and Derna in 2011 after the fall of the former Libyan leader Muammar Gaddafi 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 and three other American staff were killed when heavily-armed militants overran the US mission in Benghazi, and then attacked a nearby CIA annex in a sustained, hours-long attack.
The assault roiled the 2012 presidential campaign, with Republicans accusing Democratic President Barack Obama of being lax on security and of seeking to cover up the true cause of attack and who was behind it.
US officials have long said the attack was carried out by Islamic militants, but have denied that core Al-Qaeda leaders planned and directed the operation, as claimed by some of Obama's critics.
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 State Department spokeswoman Jen Psaki.
Nor did it mean, she added, that "those organisations preplanned the attack well in advance or that the organisations are somehow more responsible than any others involved."
Qumu was reportedly transferred from the US military jail in Guantanamo Bay, in southern Cuba, in 2007 to Libya and later released from prison.
The third group put on the US terror list was Ansar al-Sharia in Tunisia, and its leader Seifallah Ben Hassine, commonly known as "Abu Iyadh".
It is blamed for an attack on the American school in Tunis which was set on fire and badly damaged on September 14, 2012.
The arson attack "put the lives of over one hundred United States employees in the embassy at risk," the State Department said.