US Secretary of State Hillary Clinton has given evidence to the Congressional Foreign Relations Committee about what led to the attack on the US Consulate in Benghazi last year, in which four embassy staff were killed.
The State Department has been under intense scrutiny since the attack on 11 September and Mrs Clinton said she took responsibility for security failures.
Mrs Clinton told the senate the administration had nothing to hide over the handling of events in Benghazi.
Mrs Clinton will be questioned later on Wednesday in the House of Representatives.
The hearings are expected to be her last appearance on Capitol Hill as America's top diplomat.
US envoy to Libya Christopher Stevens died of smoke inhalation when he was trapped in the burning consulate building, after armed men stormed the compound.
Outrage in Congress over the incident has already led the US ambassador to the UN, Susan Rice, to withdraw from the race to succeed Mrs Clinton.
Last November, Mrs Rice admitted releasing incorrect information after the Benghazi attack. She said there had been no attempt to mislead the public, but Republicans were unconvinced.
At Wednesday's hearing, Senator Johnson (Wisconsin, Republican) said:
"We were misled that there were supposedly protests and then something sprang out of that, an assault sprang out of that."
The BBC reports, Mrs Clinton replied with a raised voice:
"But with all due respect, the fact is we had four dead Americans - 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."
Thumping the table four times, she added: "What difference, at this point, does it make? It is our job to figure out what happened and do everything we can to prevent it from ever happening again, Senator."