Two days after the deadly Libya terror attack, representatives of the FBI and National Counterterrorism Center gave Capitol Hill briefings in which they said the evidence supported an Al Qaeda or Al Qaeda-affiliated attack, Fox News has learned.

The description of the attack by those in the Sept. 13 briefings stands in stark contrast to the now controversial briefing on Capitol Hill by CIA Director David Petraeus the following day -- and raises even more questions about why Petraeus described the attack as tied to a demonstration.

The Sept. 13 assessment was based on intercepts that included individuals, believed to have participated in the attack, who were celebratory -- as well as a claim of responsibility.

FBI and NCTC also briefed that there were a series of Al Qaeda training camps just outside of Benghazi, where the attack occurred and resulted in the deaths of four Americans. The area was described as a hotbed for the militant Ansar al-Sharia as well as Al Qaeda in North Africa.

Fox News is told there was no mention of a demonstration or any significant emphasis on the anti-Islam video that for days was cited by administration officials as a motivating factor.

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Fox News is told that the Petraeus briefing on Sept. 14 conflicted with that of the FBI and NCTC.

On Capitol Hill, Petraeus characterized the attack as more consistent with a flash mob, where the militants showed up spontaneously with RPGs. Petraeus downplayed to lawmakers the skill needed to fire mortars, which also were used in the attack and to some were seen as evidence of significant pre-planning. As Fox News previously reported, four mortars were fired -- two missed the annex, but the mortar team re-calibrated and the next two mortars were direct hits.

Fox News is told that Petraeus seemed wedded to the narrative that the attack was linked to a demonstration and was spontaneous as opposed to pre-meditated.

Fox News is told that Petraeus was "absolute" in his description with few, if any, caveats. As lawmakers learned more about the attack, including through raw intelligence reports, they were "angry, disappointed and frustrated" that the CIA director had not provided a more complete picture of the available intelligence.

Reached for comment, a spokesperson for the NCTC referred Fox News to the Sept. 28 statement from the Office of the Director of National Intelligence which said the assessment changed to indicate that it did not stem from a protest, but rather was deliberate and organized. “In the immediate aftermath, there was information that led us to assess that the attack began spontaneously following protests earlier that day at our embassy in Cairo. ... Throughout our investigation we continued to emphasize that information gathered was preliminary and evolving. As we learned more about the attack, we revised our initial assessment to reflect new information indicating that it was a deliberate and organized terrorist attack carried out by extremists.”

A U.S. intelligence official disputed the characterization of Petraeus' briefing to lawmakers on Sept. 14, saying: "The first briefing (to the Hill) carefully laid out the full range of sparsely available information, with briefers noting that extremists -- including those with possible links to AQIM and Ansar al-Sharia -- were involved in attacks that appeared spontaneous. The talking points (from that weekend) clearly reflect the early indications of extremist involvement in a direct assault."

As for the current assessment of the Benghazi attack, a U.S. intelligence official said no one is ruling out the idea militants may have aspired to attack the U.S., though the bulk of available information supports the early assessment that extremists -- with ties to al Qaeda and Ansar al-Sharia -- did not plan the attacks for day or weeks in advance.

One source who heard Petraeus brief also told Fox News, "I can confirm that he explicitly stated both to the House and the Senate oversight committees that members of AQIM and AAS participated in the attack in Benghazi. That assessment still stands."

Intelligence officials have given a mixed picture of what happened that day, acknowledging that an investigation is under way. The office of Director of National Intelligence James Clapper last month acknowledged the strike was a coordinated terror attack, but officials have subsequently said the strike could have been opportunistic -- taking their cue from protests over the film in Egypt.

Still, some point to the use of mortars and several other strands of evidence to claim the attack involved some premeditation.