Hillary Clinton endured hours of sharp questioning by House lawmakers leading a select House committee investigating the Benghazi attacks in 2012 that led to the deaths of American ambassador Christopher Stevens and three fellow Americans.

During her opening statements on Thursday, the former secretary of state said she took responsibility for the terrorist attack on the U.S. consulate and then proceeded to introduce reforms to protect diplomatic staff while she was working under the Obama administration.

"After the attacks I stood next to President Obama as Marines carried his casket," she said, referring to Stevens, who she said she had asked to go to Libya as US envoy.

"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," she told the congressional committee in Washington, D.C.

However, the committee's Republicans and chairman Rep. Trey Gowdy of South Carolina argued that she had failed in her duty to secure the American diplomatic mission in Benghazi in the months before the attacks.

"Why were there so many requests for security equipment and personnel and why were those requests denied in Washington?" Gowdy demanded in his opening remarks. "What did our leaders in Washington do or not do, and when?"

Democrats, on the other hand, came to Clinton's defense, arguing that the Republican-led panel is a politically motivated witch-hunt aimed at derailing her presidential bid.

"It is time, and it is time now, for Republicans to end this taxpayer-funded fishing expedition," said Democratic Rep. Elijah Cummings, calling for a "shift from politics to policy" in the inquiry about the Benghazi attacks.

However, Gowdy insisted that the goal of the probe is to find the truth behind the attacks. The committee has also suggested they will present new evidence to show how poorly the U.S. State Department responded to security requests for the Benghazi mission, in addition to new evidence on how the military responded to the attack.

Meanwhile, Clinton maintained her argument that the requests for additional security didn't reach her level and that the State Department has since taken measures to improve security at diplomatic missions.