WASHINGTON (CNN) — Former CIA Director David Petraeus arrived on Capitol Hill on Friday to testify before Congress about what he knows about the attack last September on the U.S. Consulate in Benghazi, Libya.

A source told CNN that Petraeus knew almost immediately that it was the work of a loosely formed militia with members sympathetic to al Qaeda.

Petraeus' testimony will occur behind closed doors before Republican and Democratic members of the House and Senate intelligence committees examining the September 11 attack, which left four Americans dead, including U.S. Ambassador Chris Stevens.

The attack on the consulate, a political hot button during a presidential election year, raised questions about everything from security at the compound to the Obama administration's initial description of the events.

Petraeus' surprise resignation last week added fuel to that fire. The United States' top spy publicly admitted to an extramarital affair, and critics of the administration suggested that his resignation might be linked to fallout over the attack.

California Democratic Sen. Dianne Feinstein, the Senate Intelligence Committee chairwoman, has insisted that regardless of his resignation, Petraeus would still testify about the attack. She has said what he knows is imperative for committee members to understand events before, during and after the attack.

Petraeus is expected to tell lawmakers that the CIA knew soon after the attack that Ansar al Sharia was responsible, according to an official with knowledge of the case. The official spoke on condition of anonymity because of the sensitivity of the subject matter.

Ansar al Sharia is more of a label than an organization, one that's been adopted by conservative Salafist groups across the Arab world.

Petraeus said he believes, according to the official, that there's been confusion concerning two separate intelligence questions: Who was responsible? And what was the motivation of the attack?

According to the official, Petraeus says the stream of intelligence from multiple sources, including video at the scene, indicated the group was behind the attack.

But separate intelligence emerged at the same time indicating the violence at the consulate was inspired by protests in Egypt over an ostensibly anti-Islam film that was privately produced in the United States. The movie, "Innocence of Muslims," portrayed the Prophet Mohammed as a womanizing buffoon.

There were 20 intelligence reports that indicated that anger about the film may be to blame, the official said.

The CIA eventually disproved those reports, but not before Petraeus' initial briefing to Congress the day after the attack, where he discussed who might be behind the attack and what prompted it. During that briefing, he raised Ansar al Sharia's possible connection as well as outrage about the film, the official said.

Petraeus' aim in testifying, according to the official, is to clear up "a lot of misrepresentations of what he told Congress initially."

The former CIA director also is expected to tell the congressional committees that he did develop unclassified talking points in the days after the attack but had had no direct involvement in developing the ones used by Susan Rice, the U.S. ambassador to the United Nations.

Rice has been under fire for suggesting the attack on the consulate was a spontaneous event spurred by a protest against the anti-Muslim film.

House and Senate committee members said they got their clearest picture yet of the attack Thursday, thanks to testimony and a video compilation of footage captured by surveillance cameras and a drone. The video showed the attack as it began and contained images "through the incident and exodus," Feinstein said.

The video included shots of Stevens, the ambassador, being dragged out of the building, said a source familiar with the House committee hearing.

Another official familiar with the hearing said one Republican House member "got into it" Thursday with acting CIA Director Michael Morrell and Director of National Intelligence James Clapper about Rice's comments, challenging why they weren't as strong as they should have been on whether an extremist element was involved in the attack.

The official, who was not authorized to discuss the matter, spoke on condition of anonymity.

Petraeus, meanwhile, can also expect to be asked whether his resignation from the CIA's top post had anything to do with the attack in Libya.

Rep. Dutch Ruppersberger of Maryland, the senior Democrat on the House Intelligence Committee, told reporters Thursday that he plans to ask Petraeus: "Did your resignation have anything to do with the fact that you were supposed to testify before Congress?"

Petraeus told Kyra Phillips of HLN, CNN's sister network, that his resignation was not linked to the Benghazi attack and that he never passed classified information to the woman he was having an affair with at the time.

--
CNN's Barbara Starr, Suzanne Kelley, Ted Barrett and Chelsea J. Carter contributed to this report.

™ & © 2012 Cable News Network, Inc., a Time Warner Company. All rights reserved.