Former secretary of State Hillary Clinton will testify Thursday before the House Select Committee on Benghazi in what is sure to be the committee’s most dramatic session since it was created 18 months ago. Here’s a summary of the panel’s controversial history and a look ahead at the questions Clinton likely will face about the 2012 attacks.

A U.S. outpost comes under attack

On the night of Sept. 11, 2012, terrorists overran a U.S. diplomatic compound inBenghazi, Libya. The violence carried over into the morning of Sept. 12 and included a sophisticated mortar attack on a nearby CIA annex. In the end, four Americans — Ambassador Christopher Stevens, State Department information management officer Sean Smith, and CIA contractors Tyrone Woods and Glen Doherty — were killed.

Ahmed Abu Khattala of Libya, an extremist militia leader, has been charged with murder and is awaiting trial in the United States.

But more than three years after the attacks, the search continues for evidence that someone in the U.S. government also bears responsibility.

Why were American diplomats in Benghazi to begin with? Why was security at the compound so inadequate? Did the intelligence community miss any warning signs? Why was the military unable to get there fast enough to perform a rescue? And why were Obama administration officials initially incorrect in explaining the attacks as a spontaneous flare-up during a protest?

Congress investigates

Five House committees and two Senate committees launched probes, each with a particular focus. At the State Department, an independent Accountability Review Boardexamined internal problems at the agency, with an emphasis on diplomatic security.

Secretary of State Hillary Clinton, who was not interviewed by the Accountability Review Board, testified in January 2013 before the Senate Foreign Relations and House Foreign Affairs committees. She took responsibility for the tragedy and agreed to improve security at diplomatic outposts in high-threat areas. She said she was not involved in decisions, before the attacks, about whether to bolster security in Benghazi.

The collection of investigative reports reached harsh conclusions. Requests for extra security in Benghazi went unheeded in the weeks and months before the attacks, and there was poor communication within the State Department about those requests.National security officials did not respond to increased violence in the region by deploying more military assets nearby that could have responded sooner. After the attacks, U.N. Ambassador Susan Rice was given inaccurate information about the incident that she shared widely with the American public.

In its report, the Accountability Review Board cited “systemic failures and leadership and management deficiencies” among senior officials at the State Department. Four agency employees were reassigned.

Significantly, none of the reports found evidence that Clinton or anyone else had ordered the military not to attempt a rescue, that the CIA had been trying to move weapons from Libya to Syria, or that Rice’s misstatements after the attacks were a deliberate bid to downplay the terrorist threat for political reasons.

A select committee is born

Questions about the attacks persisted, especially from Republican members of Congress. In May 2014, House Speaker John Boehner chose Republican Rep. Trey Gowdy of South Carolina to head a special select committee on the incident.

Even then, Gowdy found himself defending the panel from allegations that it was created to damage Clinton politically.

“I’ve got to get the facts. I don’t want to sound naive, but facts do not come with a color associated,” Gowdy said in May 2014. “There are not Republican facts or Democratic facts or swing-state facts. There are just facts.”

The seven Republican and five Democratic lawmakers on the Select Committee on Benghazi got off to a cordial start. Working with a budget of several million dollars, they hired staff, requested documents and interviewed the families of the four men killed. Their first two public hearings focused on implementing the Accountability Review Board’s recommendations, especially those involving security.

Democrats questioned the need for a select committee, saying all relevant questions about Benghazi had been “asked and answered.” But the investigation continued, mostly behind closed doors, with little controversy.

Cooperation gives way to partisanship

Gowdy demanded the Obama administration turn over all Benghazi-related documents, saying they were needed for a complete record. Problems obtaining some of those documents strained the committee’s bipartisan spirit.

Gowdy and Rep. Elijah Cummings of Maryland, the committee’s top Democrat, began battling publicly over the path forward, and the panel stopped holding public hearings. Thursday’s testimony from Clinton will be the committee’s first public hearing since January.

Emails offer a new focus

The Benghazi committee obtained some of Clinton’s emails last year. In December, Clinton delivered about 30,000 additional work-related emails to the State Department.

In early March, The New York Times reported that Clinton has exclusively used a personal email account on a private server to conduct all official business while she led the department. Two days later, the Benghazi committee subpoenaed all her email correspondence related to Libya.

The revelation about Clinton’s email policy became a turning point in the investigation, especially after Clinton said she deleted about 30,000 emails on the private server because they were personal. The FBI is investigating whether the private email system was secure.

Clinton’s email policy — which Gowdy says jeopardizes the likelihood his committee will receive all relevant documents — led to records never before seen by congressional investigators. Gowdy also said the panel has interviewed witnesses never before questioned by Congress, which is why Republicans promise there’s new information to be explored, starting with Clinton’s testimony on Thursday.

What can we expect next?

The committee is investigating events before, during and after the attacks, but Gowdy said recently that most of the questions Clinton will face Thursday probably will focus on decisions made before the attacks. Those questions could involve rejected security requests, who was advising Clinton on Libya policy (and what their motives were), and what Clinton did with such advice.

A recent letter from Gowdy to Cummings reveals that Republicans are interested in the role of longtime Clinton family friend Sidney Blumenthal. In the months leading up to the Benghazi attacks, Blumenthal emailed Clinton directly about political conditions in Libya, even though he didn’t work for the federal government, according to excerpts of 2011 emails cited by Gowdy.

“Dozens of emails between Clinton and Blumenthal show an individual who tried to heavily influence the Secretary of State to intervene in Libya,” Gowdy wrote to Cummings.

Gowdy also alleges Blumenthal was “pushing Secretary Clinton to war in Libya” because he had a financial interest in a company that stood to win contracts with the new Libyan government.

Republicans also may focus on Blumenthal’s unfettered access to Clinton at a time when lower-level employees handled Benghazi-related security concerns.

Gowdy told Face the Nation on Sunday that the committee recently received additional emails from Ambassador Stevens, which have not been reviewed by any other committee. He said those emails show Stevens asked the State Department for increased security at the Benghazi compound.

“I want to know why certain things made it to your inbox, Madam Secretary, but the plaintive pleadings of our own ambassador that you put in place for more security never bothered to make it to your inbox,” Gowdy said. “I think that’s a fair question.”