Hillary Clinton is giving evidence to Congress over the deadly attack on the US consulate in Benghazi, Libya, last year.
“I take responsibility,” the Secretary of State told the foreign relations committee of the Senate, referring to security failures that led to the attack.
She had been due to testify late last year but fell ill.
Christopher Stevens, the US ambassador to Libya, and three other officials were killed in the attack on September 11, 2012.
The ambassador died of smoke inhalation when he was trapped in the burning consulate building, after armed men had stormed the compound.
The assault triggered a major political row over who knew what and when. As a result, an independent panel – the Accountability Review Board – was charged with investigating the incident.
Hillary Clinton told the Senate committee she never saw the paperwork asking for more security at the consulate in Benghazi.
“The specific security requests pertaining to Benghazi… were handled by the security professionals in the department. I didn’t see those requests, they didn’t come to me, I didn’t approve them, I didn’t deny them,” Hillary Clinton said.
In her opening remarks, Hillary Clinton pointed to the rise of Islamist militancy across North Africa and said: “Benghazi did not happen in a vacuum.”
The secretary of state’s appearance comes less than a week after a siege by Islamist militants at a gas facility in Algeria in which three Americans died.
“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,” Hillary Clinton also said.
Responding to questions, she said: “We have to recognize this is a global movement – we can kill leaders but until we help establish strong democratic institutions… we’re going to be faced with this level of instability.”
Democrats hold the majority in the Senate, where Hillary Clinton is surrounded by former colleagues and the tone is likely to be respectful.
Hillary Clinton’s voice shook with emotion as she described the moment she and President Barack Obama welcomed home the caskets of those killed in the Benghazi attacks, saying this was “personal”.
In the House, however, Hillary Clinton is expected to face much more heat.
The panel review did not blame her directly for any of the failures, but members of Congress will still want to know why she was not personally aware of requests for more security in a high-risk posting like Libya.
On Tuesday, Republican Senator John McCain said he wanted to press Hillary Clinton on where she was on the night of the attack, and what warnings there had been about deteriorating security.
“It’s been a cover-up from the beginning,” he told reporters.
Hillary Clinton is also facing questions about how the administration of President Barack Obama handled the fallout.
Three State Department employees have been fired over the Benghazi attack, and recommendations the panel made in December are already being implemented.
Hillary Clinton, who is stepping down from her post in two weeks, has spent a month recuperating from a series of ailments in December.
She was treated in hospital for a blood clot near her brain, weeks after fainting and suffering a concussion in the subsequent fall.
Barack Obama appointed Hillary Clinton at the start of his first term in 2009. She is considered a strong candidate for the Democratic nomination for president should she run in 2016.
Outrage in Congress over the Benghazi incident and its aftermath has already led US ambassador to the UN, Susan Rice, to withdraw from the race to succeed Hillary Clinton.
Last November, Susan Rice admitted releasing incorrect information after the Benghazi attack. She said there had been no attempt to mislead the public, but Republicans were unconvinced.
Barack Obama has since nominated Democratic Senator John Kerry – who is expected to be swiftly confirmed – as Hillary Clinton’s replacement.
Hillary Clinton is due to testify for 90 minutes before the committees.