Four Americans, including U.S. Ambassador to Libya Chris Stevens, were killed Tuesday as gunmen set fire to and fought security forces at the U.S. Consulate in Benghazi, Libya.

The attack came as protesters outside the compound rallied against a movie that unflatteringly portrays Islam's Prophet Mohammed. We are starting to get a clearer picture of what happened and why, but many more important and larger questions about the attack in Libya that still remained unanswered.

Who exactly is behind the attack and what was their motivation?

The attack - from people with guns and rocket-propelled grenades -- came as people were protesting an anti-Islamic video outside the U.S. Consulate in Benghazi on Tuesday night, according to official Libyan and U.S. sources. However, it's not clear whether the protesters were the ones who attacked.

U.S. sources are giving conflicting accounts about whether the attack was planned before the protest and whether the attackers used the protest as a diversion.

Sources tracking militant Islamist groups in eastern Libya say that a pro-al Qaeda group responsible for a previous armed assault on the consulate -- called the Imprisoned Omar Abdul Rahman Brigades - is a chief suspect in the attack.

The sources also note that the attack immediately followed a call from al Qaeda leader Ayman al-Zawahiri for revenge for the June death of Abu Yahya al-Libi, a senior Libyan member of the terror group.

Noman Benotman, president of the counter-extremist group Quilliam Foundation in London, told CNN, "An attack like this would likely have required preparation. This would not seem to be merely a protest which escalated."

"According to our sources, the attack was the work of roughly 20 militants, prepared for a military assault; it is rare that an RPG7 is present at a peaceful protest," Benotman said.

That analysis is supported by some U.S. sources who say the attack on the consulate is believed to have been planned. The sources say the attackers used the protest as a diversion to launch the attack, although the sources could not say if the attackers instigated the protest or merely took advantage of it.

However, one U.S. official told CNN on Thursday that intelligence information indicated that the attack wasn't premeditated.

Additionally, Tommy Vietor, a National Security Council spokesman, told CNN Wednesday night that "there is a lot of press speculation for who did this and why, but at this stage it would be premature to ascribe any motive to this reprehensible act."

U.S. intelligence officials believe that it is very unlikely that the core of al Qaeda was behind the attack, one such official said Thursday. The official did not rule out a group sympathizing with al Qaeda.

The top Republican on the House Intelligence Committee said Thursday that the strike "has all the hallmarks of an al Qaeda operation or an al Qaeda affiliate."

"One of the things that we've noticed over the last six or seven months is that al Qaeda in the Maghreb, northern Africa, have said they're really eager to strike northeastern targets. We've seen cells in Libya and Egypt starting to develop," U.S. Rep. Mike Rogers, R-Michigan, told CNN's "Starting Point."

Libyan Deputy Interior Minister Wanis al-Sharif said Wednesday that a group of heavily armed militants "infiltrated the march to start chaos." Libya's government blamed remnants of the Gadhafi regime, which was overthrown last year.

There was some speculation Wednesday about whether the 11th anniversary of the September 11, 2001, terrorist attacks had anything to do with the Benghazi assault. Secretary of State Hillary Clinton said the timing was unsettling for Americans, but it provided a reminder that "our work is not yet finished."

U.S. Sen. Dan Coats, a Republican from Indiana who is a former ambassador to Germany, said he thought the timing couldn't be ignored.

"I think its no coincidence that this happened on September 11," he said.

Was the attack planned and were proper security measures in place?

Wednesday night, U.S. officials told CNN's Suzanne Kelly that there were no actionable intelligence that this attack was being planned. But there appear to be some conflicting reports on the matter. Earlier Wednesday, sources said they believed the attack was planned, and that the protest of an obscure film that mocks Muslim faith was used as a diversion.

State Department Under Secretary Patrick Kennedy, during a briefing to Capitol Hill staff, offered his opinion that the attack was planned because of the extensive nature of the attack and the "proliferation" of small and medium weapons.

Could any of this been prevented? And what kind of security was in place at the consulate? Given that it was September 11, and there has been unrest in the area, were any measures taken to step up security at U.S. posts around the world? And if not, should there have been?

Rep. Mike Rogers, chairman of the House Intelligence Committee, told CNN's Wolf Blitzer on Wednesday night that he and other officials had never been told of chatter or any indication that something like this was about to happen at the U.S. consulate in Benghazi.

"We didn't get the warning you would hope to get in an event like this, so we could have prevented the loss of life, and you know, with a horrible tragedy for losing a U.S. ambassador, Mr. Stevens," he said. "We think that we'll be able to go back and take a look. Again, we're going to rescrub all of that."