The US State Department says it is offering a $US10 million ($A10.75 million) reward to track down anyone behind last year's brutal attack on a US mission in Libya, in which the ambassador was among four Americans killed.
"Since January of this year, the Rewards for Justice program has had a reward offer of up to $US10 million for information leading to the arrest or conviction of anyone who was involved in the September 2012 Benghazi attacks," a State Department spokesman said on Friday.
The reward was not widely publicised when it was first made available, which the State Department said was because of "security issues and sensitivities surrounding the investigation."
"Since this event happened... we've made it clear that we are committed to bringing the people who conducted this attack to justice. And we're using all the appropriate tools we have to do that," the spokesman said.
Hordes of heavily armed militants stormed the mission in eastern Benghazi on September 11, 2012 and then attacked a nearby CIA compound with mortar shells and rockets.
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Ambassador Chris Stevens was killed in the firestorm along with three other diplomatic and security staff, Sean Smith, Glen Doherty and Tyrone Woods.
Stevens, a popular diplomat and fluent Arabic speaker, was the first ambassador to be killed while on duty in three decades, and the assault shocked America to its core.
Initially, the sacking of the mission was described by US officials as having been triggered by an anti-Muslim video aired in the United States which sparked protests across the Arab world.
But it was later revealed that some of those behind the assault had links to organised Al-Qaeda extremists.
So far no one has been charged or arrested in the investigation which is being led by the FBI, while the Libyan authorities are carrying out a separate probe.
The event roiled the 2012 presidential elections, with Republicans using it to hammer Democratic President Barack Obama and his administration as being lax on security.
To this day, many Republicans insist there was a conspiracy by the administration to cover up the true events of what happened in Benghazi.
A State Department internal probe slammed "woefully inadequate" security arrangements, and set up a series of recommendations to ensure missions are better protected.
