Washington (CNN) -- At times angry and choked with emotion, Secretary of State Hillary Clinton on Wednesday took on Republican critics of her department's handling of the September terrorist attack in Libya that killed the U.S. ambassador and three other Americans, but repeatedly distanced herself from a direct role in specific situations.

"As I have said many times since September 11, I take responsibility," Clinton told the Senate Foreign Relations Committee at the first of two long-anticipated congressional hearings Wednesday on the attack that became a major issue in the November presidential election.

Conservative Republicans challenged Clinton on the lack of security at the diplomatic compound in Benghazi, where Ambassador Christopher Stevens and three others were killed, as well as the erroneous account given days later by U.N. Ambassador Susan Rice that the attack grew spontaneously from a protest over an anti-Islam film produced in the United States.

Sen. Ron Johnson, a tea party backed Wisconsin Republican serving his first term, persistently questioned Clinton about he called Rice's "purposely misleading" the American people. In response, Clinton shouted in exasperation that with four Americans dead and the focus now on preventing future security breakdowns, "what difference, at this point, does it make?"

She acknowledged the "systemic breakdown" cited by an Accountability Review Board she appointed and noted she had accepted all 29 of its recommendations, adding her department was taking additional steps to increase security at U.S. diplomatic facilities around the world.

However, Clinton also told the committee she had no direct role in requests by Stevens and other diplomats for increased security in Benghazi and elsewhere, saying: "I didn't see those requests. They didn't come to me."

Another conservative Republican, Sen. Rand Paul of Kentucky, told Clinton she should have been fired her for not reading cables from Stevens and others in Libya.

The independent report from the review board said it did not find "that any individual U.S. government employee engaged in misconduct or willfully ignored his or her responsibilities" leading up to the attack. However, one State Department official resigned and three others were placed on administrative leave after the report was released in December.

Later at Wednesday's hearing, in reference to the erroneous talking points by Rice that were aired on September 16, Clinton said she was focused at that time on ensuring the safety of U.S. personnel at other facilities where protests were taking place.

"I was pretty occupied about keeping our people safe, doing what needed to be done," Clinton said, adding "I wasn't involved in the talking points process."

In her opening statement, Clinton said the Benghazi attack didn't happen in a vacuum but was part of a "broader strategic challenge in North Africa and the wider region."

Clinton's Benghazi statement: 'Not just a matter of policy -- it's personal'

he defended her department's response, saying there was "timely" and "exceptional" coordination between the State Department and the Pentagon on the night of the attack

"No delays in decision making. No denials of support from Washington or from the military," Clinton said. The review panel's report "said our response saved American lives in real time -- and it did," she added.

Clinton also said she directed the response to the attack from the State Department that night and "stayed in close contact with officials from across our government and the Libyan government."

In addition, Clinton said she immediately took steps to beef up security at U.S. posts around the world and to implement the review panel's 29 recommendations.

Clinton made clear that the security situation in North Africa and the Middle East remained threatening in the wake of the Arab Spring upheaval, with longtime leaders ousted in Egypt, Tunisia and Libya.

The fledgling Libyan leadership turned out to be unable to fulfill traditional security commitments to the U.S. diplomatic compound, she said.

"What I found with the Libyans was willingness but not capacity," she said.

Clinton also warned that weapons from Libya have turned up in Algeria and elsewhere, adding that "this Pandora's Box if you will of weapons coming out of these countries in the Middle East and North Africa is one of our greatest security threats."

The appearance in the Senate and, later Wednesday, before a House committee, was one of the last acts for Clinton before she leaves her post as long planned.

"For me, this is not just a matter of policy," she said. "It's personal."

In reference to the return of remains of the four slain Americans, Clinton said in voice choked with emotion: "I stood next to President Obama as the Marines carried those flag-draped caskets off the plane at Andrews. I put my arms around the mothers and fathers, sisters and brothers, sons and daughters."

Democrats on the panel made a point of praising Clinton's service and noted that House Republicans have voted to cut funding for diplomatic security.

Clinton was originally scheduled to testify last month but postponed her appearance as she was treated for illness, a concussion and a blood clot near her brain. The country's top diplomat returned to work just over two weeks ago.Sen. Clinton Testifies On Benghazi Attack