More than four months after the deadly attack on the U.S. diplomatic compound in eastern Libya, Secretary of State Hillary Clinton will deliver long-awaited testimony on the historic security failure.

The secretary, after missing prior sessions before Congress due to illness, is set to take lawmakers’ questions Wednesday before a House and Senate committee. As Clinton prepares to leave the department after a busy four years, the hearing is a chance to address what is arguably the biggest controversy of her tenure.

“There’s no question there hasn’t been the accountability within the department,” Sen. Bob Corker, R-Tenn., the incoming top Republican on the Senate Foreign Relations Committee, told Fox News. “I think this is going to be the beginning, I hope … of a top-to-bottom review.”

Congress has already held multiple hearings to date, with lower-level officials testifying, on the Benghazi attack. The administration has gradually backed off its position that the attack was spontaneous — an assertion that drew the ire of congressional lawmakers who said it was clearly planned. An official State Department-sanctioned report has resulted in several officials leaving their current posts, after finding “systemic failures” left the Benghazi compound inadequately protected.

The hearing Wednesday, though, is a forum for lawmakers to question Clinton on what she might have known about requests for security at the Benghazi post — and what she thinks should be done to improve security going forward, as she prepares to leave the department. Sen. John Kerry, D-Mass., has been nominated to replace her.

State Department spokeswoman Victoria Nuland said Tuesday “there was no question” that Clinton “considered it her responsibility to learn the lessons from this, and to take the department forward in implementing whatever lessons were learned.”

There are also questions about how the Obama administration responded to the crisis during the Sept. 11, 2012 attack. Fox News has learned from senior U.S. defense officials that a FAST team of Marines out of Spain was asked by State Department officials to change out of their Marine uniforms after being asked to leave for Libya to help — this required them to deplane and delayed them by about 90 minutes, according to Pentagon officials.

Then there is the decision by Clinton and State Department Undersecretary of Management Patrick Kennedy not to mobilize the Counterterrorism Security Group, which is composed of experts on terrorism from across government agencies and makes recommendations to the deputies who assist the president’s Cabinet in formulating a response to crises involving terrorism.

Further, there are questions about the perceived delays CIA officials — stationed in Benghazi — encountered that night and their frustration that air support was not sent from nearby Sigonella air base. In recent weeks, Fox News has learned that the rescue unit that left Tripoli was told that “fast fliers” — or air support — would be above when they landed in Benghazi. They weren’t.

Still, the department’s review board concluded: “The Board members believe every possible effort was made to rescue and recover Ambassador Chris Stevens and Sean Smith. The interagency response was timely and appropriate, but there simply was not enough time for armed U.S. military assets to have made a difference.”

Lawmakers have waited months to ask Clinton direct questions about the Benghazi attack, in which U.S. Ambassador Chris Stevens and three other Americans were killed. That attack, though, may just be the starting point for Wednesday’s hearings amid mounting questions about the hostage crisis in Algeria and the French-led military campaign in neighboring Mali.

Corker told Fox News “there’s going to be a lot of questions about just the overall policies of this administration as it relates to” Al Qaeda’s affiliate in North Africa.

An Al Qaeda-tied operative has claimed responsibility for the attack at a gas plant in Algeria that left 37 hostages dead, including three Americans.

Corker suggested the developments challenge an administration that was “spiking the ball on Al Qaeda” last year as the U.S. chipped away at the group’s leadership in Pakistan and Afghanistan, and took out Usama bin Laden the year before that.

“That’s where the core of Al Qaeda (is), but there are all these nodes that are scattered through Africa now that obviously are a threat to Western civilization, and it’s as if they’ve taken their eye off the ball,” Corker said.

Clinton was preparing to testify in December, but fell ill and was unable to address Congress.

The State Department has thus far been sparing in its explanation of the separate Algeria attack.

Nuland said last week that Clinton spoke with the Algerian Prime Minister and “talked about the challenge that Algeria has had historically with terrorism.”

Clinton later said “it is absolutely essential that we broaden and deepen our counterterrorism cooperation going forward without Algeria and all countries of the region.”

She said she made clear to the prime minister that “we stand ready to further enhance the counterterrorism support that we already provide.”

Outgoing Defense Secretary Leon Panetta also had tough words, saying those who try to attack Americans “will have no place to hide.”

On the House Foreign Affairs Committee, an aide to Rep. Michael McCaul, R-Texas, also said it’s likely the Algeria hostage standoff will come up Wednesday.

Clinton plans to testify on the Senate side at 9 a.m. ET Wednesday, and at 2 p.m. on the House side.