US authorities on Wednesday released pictures of three men it said were present during the September 11, 2012 attack on the US mission in eastern Libya, saying it wanted to question them.

The Federal Bureau of Investigation's website did not say whether the three men -- all of whom appear to be carrying guns in the pictures -- are suspects in the attack that killed four Americans, including Ambassador Chris Stevens.

"The FBI is now asking Libyans and people around the world for additional information related to the attacks," the federal law enforcement agency said.

"We are seeking information about three individuals who were on the grounds of the US Special Mission when it was attacked. These individuals may be able to provide information to help in the investigation," it said.

US President Barack Obama has faced withering criticism over the attack from Republicans, who accuse the administration of failing to do enough to protect diplomatic missions and of covering up the true nature of the attack.

Senior administration officials initially described the attack as a spontaneous reaction to an anti-Islam Internet video that had sparked violent demonstrations in Cairo and elsewhere.

Officials later admitted that there had been no protest outside the consulate in the eastern city of Benghazi, and US media have reported that a nearby annex -- which was also attacked -- was part of a secret CIA mission.

US President Barack Obama has referred to the assault as a "terrorist" attack and vowed to hunt down those responsible.