WASHINGTON — Secretary of State Hillary Rodham Clinton, at times emotional and fierce, insisted on Wednesday that the department is moving swiftly and aggressively to strengthen security at U.S. missions worldwide after the deadly Sept. 11 raid on the consulate in Libya.

In her last formal congressional testimony on Capitol Hill as America's top diplomat, Clinton once again took full responsibility for the department's missteps leading up to assault at the U.S. facility in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans.

Her voice cracking at one point, Clinton said the experience was highly personal.

"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," she told the Senate Foreign Relations Committee at a jam-packed hearing.

Her voice rising at another point, she defended U.N. Ambassador Susan Rice, who was vilified for widely debunked claims five days after the attack that protests precipitated the raid rather than terrorism. She challenged the GOP focus on Rice's comments, which were based on intelligence talking points.

"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? What difference, at this point, does it make?" a clearly exasperated and angry Clinton told Sen. Ron Johnson, R-Wis. "It is our job to figure out what happened and do everything we can to prevent it from ever happening again, senator."

She insisted that "people were trying in real time to get to the best information," and that her focus was on looking ahead on how to improve security rather than revisiting the talking points and Rice's television appearance.

Clinton said the department is implementing the 29 recommendations of an independent review board that harshly criticized the department as well as going above and beyond the proposals, with a special focus on high-threat posts.

The review board report faulted "systematic failures and leadership and management deficiencies at senior levels within two bureaus of the State Department" and four employees were put on administrative leave.

"Nobody is more committed to getting this right," she said. "I am determined to leave the State Department and our country safer, stronger, and more secure."

Three weeks after her release from a New York hospital, Clinton was at times defiant, complimentary and willing to chastise lawmakers. She will appear before the committee on Thursday to introduce her likely successor, Sen. John Kerry, D-Mass.

Clinton refused to back down from withering GOP criticism of the Obama administration's shifting explanations about the assault.

Sen. John McCain, R-Ariz., a Clinton friend in the Senate, offered praise along with harsh complaints.

"It's wonderful to see you in good health and combative as ever," McCain told a visibly slimmer Clinton, whose planned testimony last month was delayed because of her illness.

In the same breath, he dismissed her explanation of events, the administration's response to warnings about the deteriorating security situation in Libya and even the attention paid to Libya after rebels toppled strongman Moammar Gadhafi.

For her part, Clinton complained about the congressional holds placed on foreign aid and bilateral assistance. "We have to get our act together," she told the panel.

Her testimony focused not only on the attack but the growing threat from extremists in northern Africa, pointing out that Libya was not an isolated incident.

"The Arab revolutions have scrambled power dynamics and shattered security forces across the region," she said. "And instability in Mali has created an expanding safe haven for terrorists who look to extend their influence and plot further attacks of the kind we saw just last week in Algeria."

She said the Obama administration is pressing for a greater understanding of the hostage-taking there and rescue effort that left three Americans dead.

Clinton parried tough questions from Republicans, offering a detailed timeline of events on Sept. 11 and the Obama administration efforts to aid the Americans in Libya while simultaneously dealing with protests in Cairo and other countries.

GOP lawmakers repeatedly questioned Clinton about whether she had seen earlier requests for beefed-up security.

"I did not see these requests. They did not come to me. I did not approve them. I did not deny them," she said.

That provoked a testy response from Sen. Rand Paul, R-Ky., a potential presidential candidate in 2016. He excoriated Clinton and expressed disbelief that she hadn't read the cables about security concerns.

"Had I been president at the time and I found that you did not read the cables from Benghazi, you did not read the cables from Ambassador Stevens, I would have relieved you of your post," Paul told Clinton. "I think it's inexcusable.

Clinton took Republicans to task, chiding House GOP members for recently stripping $1 billion in security aid from the hurricane relief bill and the Senate panel for failing for years to produce an authorization bill.

In northern California, Stevens' stepfather, Bob Commanday, said the family has avoided discussions of whether security was adequate. He said Clinton had been in contact with the family on several occasions since the attack.

"We're very aware of her sympathy because of our contact with her and the way she has connected with us and written to us," he said. "It's a tragedy and nothing that is said or done can bring him back, so we are just going on with life."

In something of a valedictory, Clinton noted her robust itinerary in four years and her work, nearly 1 million miles and 112 countries.

"My faith in our country and our future is stronger than ever. Every time that blue and white airplane carrying the words "United States of America" touches down in some far-off capital, I feel again the honor it is to represent the world's indispensable nation. And I am confident that, with your help, we will continue to keep the United States safe, strong, and exceptional."

Clinton was the sole witness at back-to-back hearings before the Senate and House foreign policy panels on the September raid.

Clinton had been scheduled to testify before Congress last month, but an illness, a concussion and a blood clot near her brain forced her to postpone her appearance.

Absent from the hearing was Kerry, the man tapped to succeed Clinton. His swift Senate confirmation is widely expected, with his confirmation hearing scheduled for Thursday. Sen. Bob Menendez, D-N.J., the incoming chairman, presided over the hearing.

Clinton's testimony was focusing on the Libya attack after more than three months of Republican charges that the Obama administration ignored signs of a deteriorating security situation there and cast an act of terrorism as mere protests over an anti-Muslim video in the heat of a presidential election. Washington officials suspect that militants linked to al-Qaida carried out the attack.

Politics play an outsized role in any appearance by Clinton, who sought the Democratic presidential nomination in 2008 and is the subject of constant speculation about a possible bid in 2016. The former first lady and New York senator — a polarizing figure dogged by controversy — is about to end her four-year tenure at the State Department with high favorable ratings.

A poll early last month by the Pew Research Center for the People & the Press found 65 percent of Americans held a favorable impression of Clinton, compared with 29 percent unfavorable.

On the panel at the hearing were two possible 2016 Republican presidential candidates — Florida's Marco Rubio and Paul, a new member of the committee.

Clinton did little to quiet the presidential chatter earlier this month when she returned to work at the State Department after her illness. On the subject of retirement, she said, "I don't know if that is a word I would use, but certainly stepping off the very fast track for a little while."