But she said she had gone along with a recommendation from subordinates that the Benghazi post be kept open and assumed that they would take the necessary steps to protect it.

Mrs. Clinton first publicly took responsibility for the Sept. 11 attack in an Oct. 15 interview with television reporters. Since then, she has committed herself to putting in place all of the recommendations of an independent review that was led by Thomas R. Pickering, a former American ambassador, and Mike Mullen, the retired admiral who served as the chairman of the Joint Chiefs of Staff.

For all of the hours of testimony, the hearings did little to clarify the role of the White House in overseeing the American presence in Libya before the attack or explain why the Pentagon had few forces available on the anniversary of the Sept. 11, 2001, terrorist attacks to respond quickly to any assault on diplomatic outposts in the region.

One of the sharpest exchanges of the day came when Mrs. Clinton responded to questions from Senator Ron Johnson, a Wisconsin Republican, by saying there was too much focus on how the Benghazi attack had been characterized in its early hours and not enough on how to prevent a recurrence. Republicans have repeatedly charged that Obama administration officials deliberately played down the attack, focusing much of their criticism on Susan E. Rice, the ambassador to the United Nations and once Mr. Obama’s choice to succeed Mrs. Clinton.

“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?” Mrs. Clinton said, her voice rising. “It is our job to figure out what happened and do everything we can to prevent it from ever happening again, Senator.”

Senator John McCain, Republican of Arizona, began his questions by saying, “It’s wonderful to see you in good health and as combative as ever,” and then proceeded to complain that she had failed to answer lingering questions.

Mr. McCain asserted that the Obama administration’s aversion to nation-building had precluded it from providing the kind of training and assistance that would have helped the fledgling Libyan government in Tripoli confront growing security threats from militants.