US diplomats testified on Wednesday that Washington could have done more to safeguard its mission in the Libyan city of Benghazi ahead of and during an attack last year that left four Americans dead.
The senior officials' statements to a Congressional committee came as President Barack Obama's White House accused Republicans of exploiting controversy over the Benghazi assault to score political points.
Gregory Hicks, the deputy head of the US mission in Libya and the first US official who was on the ground to speak publicly about what happened, provided a minute-by-minute account of the events of September 11, 2012.
He also recounted what is believed to be the final phone call from US ambassador Christopher Stevens, who was among those killed in Benghazi.
Hicks testified that he and many others knew from the start that the assault on the Benghazi compound was a deliberate act of terror, and said he was shocked when Obama's administration publicly argued otherwise.
He told the House Oversight and Government Reform Committee he was "stunned" after UN ambassador Susan Rice went on television to say the attacks were a spontaneous reaction after a demonstration against an anti-Islam video.
"My jaw dropped and I was embarrassed," he said, in what became a fiery and impassioned hearing.
Eric Nordstrom, the regional security officer at the US embassy in Tripoli in the months before the Benghazi attack, also testified.
He told lawmakers that senior State Department officials were aware of security shortcomings and argued that it was "inexplicable" that their actions were not reviewed more thoroughly.
The testimony by the "whistleblowers," as the committee's Republican chairman Darrell Issa called them, is the latest in a months-long series of hearings to put the attacks and the US response to them under a microscope.
Republicans have hammered Obama's handling of the tragedy, accusing the White House of misleading Americans in the run up to November's presidential election by portraying the attack as spontaneous.
The Obama administration, which eventually acknowledged that the attack was planned in advance, has attributed the initial account to confusion about what was happening on the ground.
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But Hicks said it was clear from the outset that an attack was in progress.
He recalled the final, frantic call from Stevens, shortly before the line went dead, in which Stevens simply told him: "Greg, we're under attack."
Republican Trey Gowdy read an email from a top State Department appointee to senior officials describing how, one day after the attacks, she told the Libyan national assembly president that the attack was an act of terrorism.
"I told him that the group that conducted the attacks, Ansar al-Sharia, is affiliated with Islamic terrorists," acting assistant secretary of state Elizabeth Jones wrote in the previously undisclosed email, according to Gowdy.
Lawmakers stressed the importance of protecting the witnesses.
"It's about time we're hearing these stories for the first time," Republican Tim Walberg said.
Hicks described how he had been pressured by administration figures, including Jones, to stop questioning the US handling of the attacks.
"The sense I got was that I needed to stop my questioning," he said.
He also said he was kept on an unreasonably tight leash by administration lawyers when Republican congressman Jason Chaffetz visited Libya, and was told not to allow himself "to be personally interviewed" by Chaffetz.
The White House argued on Wednesday that all the circumstances of the attack had been probed by a review board ordered by then-secretary of state Hillary Clinton, and its recommendations had been acted upon.
"This is a subject that has, from its beginning, been subject to attempts to politicize it by Republicans, when in fact what happened in Benghazi was a tragedy," White House spokesman Jay Carney said.
Hicks said he pressed repeatedly for a quick military response to the attack, beyond the emergency team sent from Tripoli.
The defense attache told him the nearest fighter jets were in Aviano, Italy, two to three hours away, but that there were no refueling tankers available to offer support, Hicks said.