Secretary of State Hillary Clinton has lashed out at a senator who accused the Obama administration of misleading the public over a raid on a US consulate.
A member of the Senate foreign relations committee suggested there had been a cover-up of events surrounding the assault in Benghazi.
Mrs Clinton thumped the table as she clashed with Senator Ron Johnson.
Earlier in the hearing, she took responsibility for security failures that led to the attack.
Mrs Clinton will be grilled later on Wednesday in the House of Representatives, where she is expected to face even more heat.
The US envoy to Libya, Christopher Stevens, and three other officials were killed in the raid on 11 September.
The ambassador died of smoke inhalation when he was trapped in the burning consulate building, after armed men stormed the compound.
The assault triggered a major political row over who knew what and when. As a result, an independent panel – the Accountability Review Board – was charged with investigating the incident.
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, Wisconsin Republican Sen Johnson said: “We were misled that there were supposedly protests and then something sprang out of that, an assault sprang out of that.”
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.”
Earlier, Mrs Clinton’s voice shook with emotion as she described the moment she and President Barack Obama welcomed home the coffins of those killed in the Benghazi attacks, saying this was “personal”.
She also told the Senate committee she never saw paperwork requesting more security at the consulate in Benghazi.
In her opening remarks, Mrs Clinton pointed to the rise of Islamist militancy across North Africa and said: “Benghazi did not happen in a vacuum.”
The secretary of state’s appearance comes less than a week after a siege by Islamist militants at a gas facility in Algeria in which three Americans died.
“Instability in Mali has created an expanding safe haven for terrorists who look to extend their influence and plot further attacks of the kind we saw just last week in Algeria,” Mrs Clinton said.
Three state department employees have lost their jobs over the Benghazi attack, and recommendations the panel made in December are already being implemented.
Mrs Clinton, who is stepping down from her post in two weeks, has spent a month recuperating from a series of ailments in December, which delayed her testimony.
She is considered a strong candidate for the Democratic nomination for president should she run in 2016.
Mr Obama has nominated Democratic Senator John Kerry as her replacement as Secretary of State.