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. But U.S. sources have said they believe the attack was planned before the protest, and that the attackers used the protest as a diversion.

If you're new to the story and need to catch up, here are six key things to know about the incident.

1) What happened?

On Tuesday night, protesters were outside the consulate in Benghazi, demonstrating against the film "Innocence of Muslims," which reportedly was made in California by a filmmaker whose identity is unclear.

Eventually, a group of heavily armed militants "infiltrated the march to start chaos," according to Libyan Deputy Interior Minister Wanis al-Sharif.

Initial reports indicate the four-hour assault began around 10 p.m. as gunmen opened fire on the main compound of the U.S. Consulate complex. Within 15 minutes, the gunmen entered the building.

A senior U.S. official said a rocket-propelled grenade set the consulate ablaze. American and Libyan security personnel tried to fight the attackers and the fire.

As the fire spread, three people -- Stevens, Foreign Service information management officer Sean Smith and a U.S. regional security officer -- were in a safe room, senior State Department officials said.

The three tried to leave when smoke filled the safe room. After the security officer escaped the building, he returned with others to try to rescue Stevens and Smith. Smith was found dead, apparently of smoke inhalation, officials said.

Stevens was missing. Libyans later said that bystanders found an unconscious Stevens and took him to a hospital, though U.S. officials could not confirm that account. His body was handed over to Americans at an airport; it's not clear how he died.

Two other Americans, whose names haven't been released, were killed and two others were wounded during a gunbattle between security forces and militants at the complex, a senior administration official said.

2) Who did it, and why?

National Security Council spokesman Tommy Vietor said, "At this stage it would be premature to ascribe any motive to this reprehensible act."

But sources tracking militant Islamist groups in eastern Libya say a pro-al Qaeda group responsible for a previous armed assault on the Benghazi consulate is the chief suspect. U.S. officials also said they believe the attack was not prompted by the film, but was planned beforehand.

A London think tank with strong ties to Libya speculated Wednesday that Stevens was the victim of a targeted al Qaeda revenge attack.

The assault "came to avenge the death of Abu Yaya al-Libi, al Qaeda's second in command killed a few months ago," the think tank Quilliam said Wednesday.

It was "the work of roughly 20 militants, prepared for a military assault," the think tank said, noting that rocket-propelled grenade launchers do not normally appear at peaceful protests, and that there were no other protests against the film elsewhere in Libya.

"Jihadists will want the world to believe that the attack is just a part of the protests against an amateur film produced in the U.S., which includes crude insults regarding the Prophet Mohammed. They will want the world to think that their actions represent a popular Libyan and wider Muslim reaction; thus, reversing the perception of jihadists being outcasts from their own societies," Quilliam president Norman Benotman said.

The significance of the timing of the attack, which fell on the 11th anniversary of the September 11 attacks on New York and Washington, is unclear.