Officials in Washington believe the attack that killed the US ambassador and three other diplomats in Benghazi may have been planned in advance.
US government aides told the Reuters news agency they suspect a militant Al Qaeda-linked group known as Ansar al Sharia was involved.
Ambassador Chris Stevens and his three colleagues died on Tuesday when gunman stormed the American consulate in the eastern Libyan city.
London-based think-tank Quilliam said it was retaliation for an American drone attack earlier this year that killed Al Qaeda’s deputy leader, who was a Libyan national.
The militant group on Monday urged Abu Yahya al-Libi’s fellow countrymen to avenge his killing.
US consulate attack 'was pre-planned'