Republicans escalated their charges against the Obama administration for alleged failures in handling last month’s Libya attack, thrusting the death of the U.S. ambassador there into the center of the presidential campaign Friday.

Mitt Romney accused his rival of “doubling down on denial” about the true origins of the Sept. 11 siege in Benghazi, which senior Obama officials initially said appeared to be an outgrowth of anti-American protests, not terrorism. Whether the administration has truthfully disclosed what it knew about the perpetrators of the attacks became a flash point in Thursday night’s debate, when Vice President Biden blamed the administration’s shifting explanations on U.S. intelligence agencies.

“There were more questions that came out last night because the vice president directly contradicted the testimony of State Department officials,” Romney told supporters Friday at a campaign event in Richmond. “American citizens have a right to know just what’s going on.”

Earlier in the week, State Department officials had said that they had not received reports of protests outside the compound before the killings. But Biden said the explanation that the attacks grew from a protest — rather than from terrorists determined to hit the consulate — persisted “because that’s exactly what we were told” by intelligence officials.

After trying for weeks to portray the deaths in Benghazi as part of a larger failure of Obama’s foreign policy, Republicans now sense their first real opening on national security, an area that has long been considered one of the president’s strengths. It was an unexpected twist that the administration seemed to grow more vulnerable following the debate between Biden, a veteran of global diplomacy and the former chairman of the Senate Foreign Relations Committee, and Ryan, a relative novice whose expertise is the domestic budget.

Obama officials moved swiftly Friday to blunt further fallout, first during the White House briefing and later in a speech by Secretary of State Hillary Rodham Clinton.

“There is nobody in the administration motivated by anything other than trying to understand what happened,” Clinton said. “We do not have all the answers. No one in this administration has ever claimed otherwise.”

In Thursday’s debate, Republican vice-presidential nominee Paul Ryan raised the possibility that the White House had blamed the attacks on anti-American protests at first because a successful terrorist attack would blemish Obama’s national security credentials. The successful killing of Osama bin Laden is a stock Obama campaign theme, and he frequently tells audiences that his administration has al-Qaeda “on its heels.”

“Look, if we are hit by terrorists, we’re going to call it for what it is — a terrorist attack,” Ryan said. Romney continued that theme Friday, telling voters at a campaign rally that he would investigate the varying accounts.

There are multiple investigations into the attack: an FBI probe into the deaths of the four Americans, an independent inquiry by a panel appointed by Clinton and at least two congressional probes.

As Republicans pounced on Biden’s remarks, White House press secretary Jay Carney and Clinton said Friday that there was no selective use of intelligence about the attacks.

“We have been very transparent about what we know,” Carney said. “As hours and days and weeks have passed by and more facts have come to light and more has been revealed through the investigations underway, that, you know, we have gained a clearer picture of what happened and what did not happen.”

Administration accounts have evolved because information about the attacks has evolved, Clinton said.

“We are providing the best information we have at that time and that information continues to be updated, it also continues to be put into context and more deeply understood through the process we are engaged in,” she said.

Another point of contention is whether the White House should have been alerted to requests by security officials for more manpower ahead of the Benghazi attack. Biden suggested on Thursday night that the security requests had not made their way back to Washington — a claim that was quickly contested.

Two former security officials testified before Congress this week that they had made requests for more manpower that were either ignored or rejected. Senior State Department officials acknowledged the requests but said there was no evidence that additional security would have prevented the attack. On Friday, administration officials clarified that what Biden said during the Thursday debate — that “we did not know they wanted more security” — meant only that the White House had not been informed of the threat by the State Department, not that no one in the administration had been informed.

“There was no actionable intelligence regarding the Benghazi facility” that could have been used to prevent the attack, Carney said, and continued questions about what the administration knew are “an effort here to politicize this, to turn this into an issue in the campaign.”

Carney and congressional Democrats have said some of the Republicans now criticizing embassy security had voted to cut the State Department’s security budget. Republicans counter that Democrats also voted for smaller security budgets.

In separate remarks Friday, Clinton repeatedly vowed to find out what happened and to do everything possible to protect diplomats in the field. But she said the business of diplomacy, like combat, involves a calculated risk.

“We will not retreat,” she said in a speech at the Center for Strategic and International Studies, a Washington think tank.

“We will never prevent every act of violence or terrorism, or achieve perfect security,” Clinton said. “Our people can’t live in bunkers and do their jobs. But it is our solemn responsibility to constantly improve, to reduce the risks our people face and make sure they have the resources they need to do their jobs.”

She spoke a day after a Yemeni security official at the U.S. Embassy in Sanaa was killed on his way to work.

Ed O’Keefe contributed to this report from Richmond. David Nakamura contributed from Washington.