At two Congressional hearings, which together totalled more than five hours, Clinton, 65, acknowledged a "systemic breakdown" cited by an independent review of issues leading up to the armed assault.
"As I have said many times since September 11, I take responsibility. Nobody is more committed to getting this right. I am determined to leave the State Department and our country safer, stronger, and more secure," she defended the Obama administration against allegations of a cover up of the attack at the US Consulate in Benghazi that killed three American nationals including US Ambassador Christopher Stevens.
"For me, this is not just a matter of policy, it's personal," Clinton said yesterday in her last Congressional hearing before she steps down as Secretary of State.
At the hearings, Republican members challenged Clinton on the lack of security at the US diplomatic compound in Benghazi as well as the erroneous account that the attack grew spontaneously from a protest over an anti-Islam film produced in the United States.
At yesterday's Senate hearing, Senator Ron Johnson, a Wisconsin Republican, said: "We were misled that there were supposedly protests and then something sprang out of that, an assault sprang out of that."
Clinton replied with a raised voice: "But with all due respect, the fact is we had four dead Americans - was it because of a protest, or was it because of guys out for a walk one night who decided they'd go kill some Americans."
Thumping the table four times, she added: "What difference, at this point, does it make? It is our job to figure out what happened and do everything we can to prevent it from ever happening again, senator."
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The Congressional testimony of Clinton on Benghazi was initially scheduled to be held in December, but had to be postponed due to her month-long illness during which she was briefly hospitalised due to blood clot in her head.
Clinton leaves the State Department after serving four years at its helm.
"It has been one of the great honours of my life to lead the men and women of the State Department and USAID. Nearly 70,000 serving here in Washington, more than 270 posts around the world," she said.
"Today, after four years in this job, travelling nearly a million miles, visiting 112 countries, my faith in our country and our future is stronger than ever," Clinton told lawmakers.