Hillary Clinton has said she took responsibility after an attack on a US consulate in Libya in 2012 that killed four Americans.
She told a Congressional committee that as secretary of state she introduced reforms to protect diplomatic staff after the attack.
Committee chairman Trey Gowdy said: "We owe them [the victims] the truth."
Mrs Clinton's party says the Republican-led panel is a witch-hunt trying to harm her presidential bid.
In her opening statement at the US Congress in Washington, she said she had asked Chris Stevens, one of the four killed, to go to Libya as US envoy.
"After the attacks I stood next to President Obama as Marines carried his casket," she said.
"I took responsibility, and as part of that before I left office I launched reforms to better protect our people in the field and help reduce the chance of another tragedy happening in the future."
Mrs Clinton said her appearance was her way of honouring the lives lost, and she called on those present to put national security above partisan politics.
Democratic Representative Elijah Cummings, seated next to the chairman, said Republicans were wasting taxpayer money in an effort to derail Mrs Clinton's presidential campaign.
AP
But Mr Gowdy denied his committee was going after Mrs Clinton.
Instead, he said, it was looking for the truth behind the diplomatic compound's unsuccessful requests for more security and personnel in the build up to the attacks.
At the hearing - Barbara Plett Usher,
Mrs Clinton is defending her record before, during and after the Benghazi attacks largely as she has before - taking responsibility for the mistakes, talking about lessons learned, and owning her policy positions.
The new element is her email record, which she has downplayed. She told lawmakers that most of her work on Libya and Benghazi wasn't done by email, mentioning an occasion when an intelligence report had to be carried to her by briefcase and read on the spot.
The tone has been largely measured, and the testiest exchange didn't actually involve Mrs Clinton.
Committee chairman Trey Gowdy has latched onto the role of a Clinton friend, Sidney Blumenthal, who sent her numerous intelligence reports on Libya by email.
He's suggesting Mr Blumenthal unduly influenced her Libya policy and had greater access to her than her Benghazi ambassador, Chris Stevens, who was killed.
Democratic committee members erupted in denial and demanded Mr Blumenthal's testimony transcripts be released, while Mrs Clinton looked on detached and bemused
There have already been seven congressional investigations into the attack, by suspected Islamist militants, on the US compound in the Libyan city of Benghazi on 11 September 2012.
The raid led to the deaths of ambassador Chris Stevens and three other US embassy staff.
Mrs Clinton said Mr Stevens was well aware of the risks of his job but withdrawing American presence from Libya would have been a mistake, she said.
"To retreat from the world is not an option. America cannot shrink from our ability to lead."
When the US pulls out of places, extremists gain a foothold, she added, although she did admit that security requests made by the Benghazi consulate were not met.
An independent review board found deficiencies within the State Department that the Obama administration has tried to fix, said the former first lady.
Reuters
Reuters
Observers say this is a key moment for Mrs Clinton, who solidified her position as the Democrats' frontrunner for presidential candidate after her potential rival, Vice-President Joe Biden, ruled himself out of the race on Wednesday.
Her appearance before the House of Representatives Select Committee on Benghazi could further strengthen her position or raise doubts about her suitability as a presidential nominee.
The Republicans on the committee are also under pressure to prove they have good reason to bring Mrs Clinton before them, and are not just using it as an excuse to undermine her presidential candidacy.
One thing that was unearthed by the committee's work was the former secretary of state's use of a private email server, which has dogged her 2016 campaign ever since.
Key questions
What are the possible consequences for Clinton?
The committee does not have the power to punish but depending on the conclusions there could be political consequences for Hillary Clinton, which might affect her presidential campaign
What did Clinton know about security requests?
She has said in previous testimony that requests for additional security didn't reach her level. But she has taken responsibility for the failure. And the State Department review laid out steps to improve security at diplomatic missions.
What did the other Congressional committees find?
This is the eighth congressional investigation. Previous committees criticised Mrs Clinton and the Obama administration on a number of points, especially the poor security at the Benghazi mission. And the intelligence was flawed.
What is there left to discover?
The committee has suggested there will be new evidence of how poorly the state department responded to security requests for the Benghazi mission, and new evidence on how the military responded to the attack.
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