The US quietly offered as much as $10m (£6.2m) for information in the months following the attack.
In subsequent years, the incident has become a political lightning rod, with Republicans accusing Mr Obama’s administration of covering up the involvement of militant groups in the days after the attack in order to assist Mr Obama’s 2012 re-election campaign.
In May, the Republican-led House of Representatives voted to establish a committee to investigate the White House response to the attack.