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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."
Five key questions on the latest Benghazi hearing
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.
Image copyright AP Image caption Republican Trey Gowdy and Democrat Elijah Cummings questioned Clinton - but in very different ways 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, BBC News
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 compou
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."
Five key questions on the latest Benghazi hearing
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.
Image copyright AP Image caption Republican Trey Gowdy and Democrat Elijah Cummings questioned Clinton - but in very different ways 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, BBC News
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 compou