Sufyan Ben Qumu, a 53-year-old Libyan citizen once considered a "threat" to the United States, is believed to have orchestrated or at least been involved in the Benghazi rocket-propelled grenade attack that killed U.S. Ambassador to Libya Christopher Stevens and three other State Department officials on the 11th anniversary of the Sept. 11, 2001, attacks, the sources told the network.
The report didn't say how bin Qumu may have been explicitly involved in the attack, although his fighting group, Ansar al-Shariah, is suspected of taking part.
His Guantanamo files indicat he had ties to the financiers behind the 2001 attacks, which killed nearly 3,000 people, Fox News said.
Qumu was held in the U.S. military's Guantanamo Bay detention camp in Cuba until the Bush administration released him to Libya Sept. 28, 2007, on the condition Libya keep him in jail.
The regime of leader Moammar Gadhafi later freed him from Tripoli's notorious Abu Salim prison in an amnesty program as part of a reconciliation effort with Islamists.
In 2011 Qumu led a band of anti-Gadhafi fighters in his hometown of Derna, 180 miles northeast of Benghazi, during the Libyan civil war and was involved with the Libyan Islamic Fighting Group, which played a key role in deposing Gadhafi and allying itself with the National Transitional Council, declassified files indicate.
The Benghazi-based council was Libya's de facto government during and after the war, until Aug. 8.
But the fighting group, which used the name Libyan Islamic Movement in the civil war, is banned by the United Nations as an al-Qaida affiliate. The group denies such ties.
The Fox News report came as the top U.S. counter-terrorism official called the deadly Benghazi assault an act of terrorism -- the first time the administration used that word to describe the attack, which came amid protests in Benghazi and Cairo after word circulated of an Internet video considered offensive to Islam.
"Yes, [Stevens and the three other State Department officials] were killed in the course of a terrorist attack on our embassy [consulate]," National Counter-terrorism Center Director Matthew Olsen told a Senate hearing Wednesday.
"We are looking at indications that individuals involved in the attack may have had connections to al-Qaida or al-Qaida's affiliates, in particular, al-Qaida in the Islamic Maghreb," he said.
The Islamic Maghreb al-Qaida affiliate is a radical Islamist organization linked to militant efforts to overthrow the Algerian government and institute an Islamic state. It supported the effort to topple Gadhafi. It also has an alleged longstanding involvement with smuggling, protection rackets and money laundering in Libya, the non-profit International Crisis Group reported.
"The picture that is emerging is one where a number of different individuals were involved" in the Benghazi attack, Olsen told the Senate Homeland Security and Governmental Affairs Committee.
Olsen was questioned at the hearing about the adequacy of diplomatic security in Libya.
"The attack in Benghazi was not a black swan but rather an attack that should have been anticipated," said Sen. Susan Collins of Maine, the top Republican on the panel.
The black swan theory describes a major-impact event that surprises the recipient or observer, and after the fact is often inappropriately rationalized with the benefit of hindsight.
Collins said Marines should have been sent to Benghazi to provide security months earlier as dangers grew. Last week's assault was the sixth by militants in Benghazi since April.
Olsen said intelligence agencies issued reports about the area's growing unrest, but his center had no specific indications of a new Sept. 11 attack.
After the attacks in Benghazi and Cairo, the Obama administration moved Marine units to Libya and Egypt.
The State Department began airlifting armored vehicles and other equipment to its missions in Tunisia and Sudan Wednesday.