Secretary of State Hillary Clinton began her long-awaited testimony Wednesday on the Benghazi terror attack by taking "responsibility" -- but she used the full day of hearings to repeatedly deny involvement in key controversies and pointedly reject the allegations of Republican lawmakers.

The at times heated testimony before House and Senate committees, likely to be the outgoing secretary's last, elicited praise from Democrats and frustration from Republicans. Far from putting the issue to rest, the testimony further fueled a debate that has raged on Capitol Hill for four months.

Though she said officials are following some "very promising leads" on the terrorists, Clinton herself acknowledged there are still several open questions about what prompted the attack that night.

The secretary battled tough criticism from lawmakers throughout the day. In one of the final jabs of the session, Rep. Jeff Duncan, R-S.C., told her, "Madame Secretary, you let the consulate become a death trap."

The biggest flash point Wednesday came during morning testimony.

Republican Wisconsin Sen. Ron Johnson claimed the department could have "easily" determined what happened that night by interviewing staffers who were evacuated. He was referring to the administration's initial claim that the attack sprung out of a protest. It was later determined there was no protest on the ground in Benghazi. Diplomatic security agents said as much to the FBI during interviews on Sept. 14, despite administration claims to the contrary two days later.

"We were misled that there were supposedly protests and something sprang out of that," Johnson said. "The American people could have known that (there was no protest) within days, and they didn't know that."

At that point, Clinton began to raise her voice.

"With all due respect, the fact is we had four dead Americans," she said.

"I understand," Johnson said.

Clinton continued to speak, raising her voice and gesturing: "Was it because of a protest or is it because of guys out for a walk one night and they decide they go kill some Americans?

"What difference, at this point, does it make?"

Clinton, lowering her voice, then said it is the administration's job to "figure out what happened" and prevent it from happening again.

Later in the testimony before the Senate Foreign Relations Committee, Clinton acknowledged the administration did not have a "clear picture" of what happened in the immediate aftermath. She said perhaps officials didn't do a good enough job explaining that they "didn't have a clear picture."

But Clinton still said the motivations of the attackers, to this day, are not clear. "Even today there are questions being raised," she said, referring to findings in the classified version of a recent report that she could not describe in detail.

Clinton, throughout the hearing, walked a fine line between taking responsibility generally for what went wrong and challenging specific allegations against her department and the administration.

During the opening of the hearing, Clinton said she has "no higher priority" than the security of her department's staff, and that she is committed to making the department "safer, stronger and more secure."

"As I have said many times, I take responsibility, and nobody is more committed to getting this right," Clinton said, later choking up when describing how she greeted the families of the victims when the caskets were returned.

Clinton went on to deny having ever seen the requests for more security from the Libya team that were denied by officials within the State Department.

"I didn't see those requests, they didn't come to me," Clinton said, adding those kinds of requests wouldn't normally come to the secretary.

Sen. Rand Paul, R-Ky., later suggested lives could have been saved if Clinton were more involved in reviewing security requests.

He said that if he were president, "I would have relieved you from your post."

Sen. John McCain, R-Ariz., after the exchange with Johnson, said he was not satisfied with the secretary's answers, complaining that the public still doesn't have answers on what happened.

Clinton, while pushing back against Johnson, also said she was not involved in crafting the controversial statements that U.S. Ambassador to the U.N. Susan Rice made on Sept. 16 -- in which she asserted the attack was "spontaneous" and linked to a protest.

"I wasn't involved in the talking points process," Clinton said, though she said she wasn't aware of anything that would have "contradicted" the information Rice had at the time. She noted that "going on the Sunday shows is not my favorite thing," a possible reference to claims that she declined to go on television Sept. 16.

Clinton also defended the administration's actions on the night of Sept. 11, when the U.S. diplomatic compound in Benghazi came under fire and four Americans died.

"I directed our response from the State Department and stayed in close contact with officials from across our government and the Libyan government," she said. "No delays in decision-making. No denials of support from Washington or from our military."

Citing the findings of a review panel, she said: "The board said the response saved American lives in real time -- and it did."

Several accounts relayed to Fox News, though, suggest possible delays in the response.

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 on the 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 air support would be above when they landed in Benghazi. It wasn't.

During the hearing on the House side Wednesday afternoon, Clinton was also pressed on why she was never interviewed by the State Department-sponsored board that investigated the incident. Rep. Ileana Ros-Lehtinen, R-Fla., said that was "outrageous."

Clinton said the board thought she was not "relevant" to their investigation because they were focusing on security officials. She said she "gladly" would have spoken with them.

She also addressed concerns about the four State Department officials who were removed from their jobs in the fallout from the attack - but were not removed from the department. She suggested federal law restricts what disciplinary measures could be taken against them.

The hearing Wednesday comes amid a broadening threat to U.S. interests across North Africa. There are reports that some of the attackers who took hostages in the deadly raid on an Algeria gas plant may have also participated in the Libya attack.

"Benghazi did not happen in a vacuum," Clinton said Wednesday, while saying later she could not verify that specific claim. She said instability has created an "expanding safe haven for terrorists" who plot into Algeria and other countries.

Sen. Bob Corker, R-Tenn., top Republican on the Senate committee, lamented "the spiking of the ball and the thinking that when Usama bin Laden was gone that was the end of Al Qaeda."

"We know nothing could be further from the truth," he said.

Clinton appeared to agree that the terrorist threat is far from diminished, saying Al Qaeda "wannabes" and "affiliates" continue to pose a challenge.