The 2012 attack on a U.S. compound in Benghazi, Libya, killed four Americans and caused a slew of political fallout, some of which is still reverberating today.
On Thursday, Democratic front-runner Hillary Clinton will testify about the attack in front of a Republican-led House committee — and what she says about it could have major implications for her White House bid.
Here is what you need to know about the attack itself and the investigation that followed it, plus how it affects Clinton, who was secretary of state at the time.
What happened in Benghazi?
On Sept. 11, 2012, after night had fallen, attackers scaled the wall of a U.S. diplomatic outpost in the Libyan port city of Benghazi. Clinton would later be called upon by House committees to testify about what happened next — although at first, those details were a little murky.
What we now know, from multiple congressional investigations, is this: The Benghazi compound was not heavily reinforced, and a group of attackers opened a gate, enabling dozens of armed men to enter. The men set fire to the building, filling it with smoke. U.S. Ambassador to Libya Chris Stevens — who was visiting Benghazi that day — scrambled to a safe room with fellow American Sean Smith, a State Department communications specialist, after American security guards attempted and failed to evacuate them. Hours later, the two were found inside the building and rushed to a local hospital, but both died of smoke inhalation.
Meanwhile, American forces from the compound, including a CIA support team who had rushed over to assist with defending against the attack, retreated to a nearby CIA annex. The annex then was attacked twice, and two more Americans were killed: CIA security contractors Tyrone Woods and Glen Doherty, who had been defending the building from the rooftop. Security officials who came from the Libyan capital of Tripoli, along with Libyan military troops, helped evacuate the rest of the U.S. personnel there.
All told, the attack happened over the course of about eight hours at the two locations.
What sparked the attack?
This has been a point of contention. The attack came the year after Libya's civil war, which was part of a larger protest movement across the Middle East and North Africa.
On that day, demonstrators in other Muslim countries were protesting outside U.S. embassies in response to an amateur film, "The Innocence of Muslims," which had recently been translated to Arabic and expressed anti-Islam sentiment.
Susan Rice, then-U.S. ambassador to the United Nations, said on Sunday talk shows that week that the "current best assessment" was the attack was an extension of the protests against "The Innocence of Muslims" happening elsewhere, including the U.S. Embassy in Cairo.
"That's the best information we have at present," Rice told ABC News' Jake Tapper.
She was telling the truth — a CIA assessment in those early days had indeed found that was the case, but that information turned out to be wrong.
The Benghazi attackers, then-acting CIA Director Michael Morell later said, were actually a ragtag, disorganized group of militants who decided at the spur of the moment to storm the compound amid the chaos in Libya following its civil war. One has been apprehended: Ahmed Abu Khattala was captured in June 2014 by U.S. special forces and brought to the U.S. to be tried.
While congressional investigations determined the initial CIA review was an honest mistake, Republicans have seized the switch in explanation as an opportunity to blame the Obama administration of a cover-up for failing to protect Americans. The controversy reached such high fever pitch, it forced Rice to withdraw her name from consideration for secretary of state.
It also led to the discovery of Hillary Clinton's use of private email servers, and more recently, killed House Majority Leader Kevin McCarthy's hopes of becoming the new Speaker of the House after he publicly linked the committee investigating the attack with Clinton's faltering poll numbers, implying the committee's findings were motivated by politics.
How is Hillary Clinton involved?
At a January 2013 Senate hearing, an exasperated Clinton asked, "Was it because of a protest or was it because of guys out for a walk one night and decided they'd go kill some Americans? ... At this point, what difference does it make?"
The difference, opponents have argued, is whether or not the attack was preventable. Republicans contend the Obama administration didn't respond appropriately to intelligence warnings before the attack and then covered up what had happened. While independent investigations have disproved that, the controversy has haunted Clinton — and only got worse when the House Select Committee on Benghazi, created by former Speaker of the House John Boehner, discovered in March that Clinton had used unauthorized, private email servers for official State Department communication.
That scandal continues to have serious consequences for Clinton. An NBC News/Wall Street Journal poll released this week found 47 percent of Americans feel the use of her private email server is an important factor in deciding their vote. And 44 percent of Americans say that they're not satisfied with Clinton's response overall to the Benghazi attack when she was secretary of state. That includes 77 percent of Republican respondents, 40 percent of independents, and 14 percent of Democrats.
What's the committee looking for?
The House Select Committee, according to the Associated Press, has interviewed 54 witnesses, including seven eyewitnesses to the attack who hadn't been interviewed in earlier investigations. It's also reviewed more than 50,000 pages of documents never before seen by Congress, including emails from Ambassador Stevens and other top State Department personnel, and received some 7,000 printed pages of emails involving Stevens.
Committee chairman Rep. Trey Gowdy, R-S.C., has said he plans to ask Clinton on Thursday why U.S. security in Benghazi wasn't bolstered as conditions across Libya deteriorated, especially in light of the fact that other countries, such as Britain, were pulling their staff out at the time.
In previous testimony, Clinton has said that requests for more security did not make their way up the chain to her.
Gowdy and other Republicans have complained that the State Department has resisted their efforts by slow-walking emails and other documents needed to complete the inquiry. Democrats counter that the $4.5 million inquiry is a costly partisan hunt to destroy Clinton's White House bid and complain that they have been frozen out of some of the committee interviews.
The committee will issue a final report on what contributed to the attacks and how the U.S. responded. Its work is expected to continue into 2016, the year of the presidential election.