Hillary Clinton came face-to-face with the special investigation of the 2012 attacks on a US diplomatic mission in Libya.
Mrs Clinton was hoping to put to rest the worst episode of her tenure as secretary of state and clear an obstacle to her presidential campaign.
Democrats have criticised the investigation as a ploy to derail Mrs Clinton’s White House bid.
Mrs Clinton, the front-runner to win the Democratic nomination, kicked off a long day of questioning with a plea that the United States maintains its global leadership role despite the threat posed to US diplomats.
She hailed the efforts of the four Americans who died in the September 11 2012 attacks, including the first ambassador in more than three decades, but told the House Benghazi Committee that the deadly events have already been exhaustively scrutinised.
The hearing comes at a moment of political strength for Mrs Clinton. On Wednesday, a potential rival for the Democratic nomination, vice president Joe Biden, announced he would not join the race. Mrs Clinton is also riding the momentum of a solid debate performance last week.
The Benghazi panel’s Republican chairman says he will not call Mrs Clinton to appear a second time.
But Trey Gowdy of South Carolina started the hearing with a series of questions that he said remained unanswered: why was the US in Libya; why were security requests denied; why was the military not ready to respond quickly on the anniversary of September 11 and why did the Obama administration change its story about the nature of the attacks in the following weeks?
Mrs Clinton addressed some of these matters in her opening remarks. She stressed a need for diplomats to advance US interests in the world, even in dangerous places, and said perfect security can never be achieved.
The committee’s top Democrat, Elijah Cummings of Maryland, described the entire probe as a partisan campaign replete with implausible conspiracy theories.
The Republican criticism has included contentions that Mrs Clinton personally denied security requests and ordered the US military to “stand down” during the attacks, or that her agency was engaged in an elaborate gun-running scheme in eastern Libya. None of these were substantiated in the independent Accountability Review Board investigation ordered by Mrs Clinton after the incident, and seven subsequent congressional investigations.
Mr Gowdy is engaged in his own balancing act, portraying the panel as focused on the facts after comments by fellow Republicans describing the inquiry as an effort designed to hurt Mrs Clinton’s presidential bid. Democrats have pounced on the remarks, and pointed out that the probe has now cost US taxpayers more than 4.5 million US dollars and, after 17 months, has lasted longer than the 1970s Watergate investigation that brought down president Richard Nixon.
The hearing is expected to last the whole day, with breaks. Mrs Clinton is certain to face questions about her use of a private email account and server while serving as President Barack Obama’s chief diplomat, another issue that has beset her campaign.
A new Associated Press-Gfk poll shows the public mixed on Mrs Clinton and Benghazi. Four in 10 say they neither approve nor disapprove of how she has answered questions about the attack, while 20% approve and 37% disapprove.
Americans also are divided on Mrs Clinton’s emails, which have raised security concerns. More than half of those polled view her use of a private server as a minor problem or no problem at all, compared with one in three who think it is a major problem. Nearly two-thirds of Republicans see it as a major problem.
In the weeks after Benghazi, Mrs Clinton took responsibility for the incidents. In recent months, she has said her use of a private email server was a mistake.