One of the biggest questions still outstanding about the attack on a United States consulate in Libya is whether it was planned or whether it was the result of a protest against a U.S.-made film that criticizes the Prophet Muhammad.

The attack killed four Americans, including Ambassador Chris Stevens.

The bottom line is that nothing is firm. But NPR's Leila Fadel reports that Libya's Deputy Interior Minister, Wanis al Sharef, said this was a sophisticated two-prong attack.

The first attack was against the consulate in Benghazi and the second came hours after the original attack, when American and Libyan forces were attempting to rescue two Americans from a safe house.

"Militants staged a second attack on the safe house," Leila reports. "Sharef said it may have been an infiltrator inside the Libyan security forces that tipped the militants off to the location of the safe house. He says this is the attack that killed the two unidentified Americans and wounded 14 others."

It's important to note that U.S. officials from the State Department, the National Security Council and law enforcement have told NPR that they have not been able to confirm that this was a planned attack.

The Associated Press also spoke to Sharef who told them that the "attacks may have been timed to mark the 9/11 anniversary" and that the "militants used civilians protesting an anti-Islam film as a cover for their actions."

Leila reports that part of what Sharef said is contradicted by witnesses.

"A lot of the witnesses that we've spoken to — neighbors, the son of the landlord, a Libyan guard who was wounded during the first part of the attack on Tuesday night — all say there was no protest at all," Leila reports. "They say that it began as an organized attack on the consulate."

Update at 6:42 p.m. ET. Avenge An Al-Qaida Death:

The experts at the Quilliam Foundation, a "counter-extremism think tank" based in London, have put out their own reading of the situation.

"We have reason to believe that the attack on the U.S. consulate in Benghazi came to avenge the death of Abu Yaya al-Libi, al-Qaida's second in command killed a few months ago," the group writes in a press release.

They report that according to their sources, the attack was perpetrated by about 20 militants and it happened 24 hours after an al-Qaida called on his followers to avenge the death of al-Libi.

House Intelligence Committee Chairman Mike Rogers echoed that thinking in an interview with Fox News.

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