WASHINGTON - During her opening statement before a Senate health committee Tuesday, Rochelle Walensky, director of the Centers for Disease Control and Prevention, was adamant that the Biden administration's masking and social distancing policies remained sound.

"While we continue to have community transmission," she told a bipartisan panel of senators who were respecting the social distancing rules of the moment as their masked staffers looked on, "we must also maintain public health measures we know will prevent the spread of this virus: mask hygiene, hand hygiene and physical distancing."

Even under hostile questioning from Republican senators, including Sen. Bill Cassidy, R-La., Walensky ticked off a series of statistics to support the CDC's current guidance. She noted that only a third of the country was fully vaccinated, stressed the high rates of cases in many counties and ended with a vague comment that the agency works to review and update their formal advice.

Left unsaid by Walensky was that she had already made a decision the night before - approving a recommendation Monday from CDC officials to significantly overhaul its guidance to no longer require fully vaccinated individuals to wear masks or physically distance in most cases. As she batted away questions from senators demanding to know when Americans could ditch the masks, scientists back at the CDC were performing one last scrub of the data and studies to identify the roughly 700 CDC webpages that needed to be updated with the new guidance.

The revised guidance - which was not publicly announced until Thursday - marked a dramatic turning point in the nation's 14-month battle with the coronavirus pandemic and sent the clearest signal yet the country could begin returning to normal. It was a significant acceleration of the schedule President Joe Biden had set earlier in the month, saying that he expected the country to move toward normalcy on July 4, when he hopes that 70% of the country's adult population will have at least one shot of the vaccine.

But the huge policy turnaround caught senior White House and administration officials, medical experts, elected officials and business leaders completely off guard, and prompted some physicians to criticize the move as premature. Some Democratic governors were angered by the White House's rollout, arguing the move effectively passed the buck to states and businesses to implement the new rules without any assistance.

The abrupt timing of Walensky's decision also smacked of politics to Biden's antagonists, who noted that the president benefited from the announcement during a difficult week when many Americans queued up in gas lines, tensions in Israel flared and markets roiled amid inflation fears.

The White House vigorously denied any interference in the decision. Instead, administration officials said, part of the communications stumble arose from the White House's hands-off policy toward the CDC as it seeks to restore public trust in the agency after it faced unprecedented political interference under the Trump administration.

"As they have done throughout the Biden administration, the CDC operates and makes decisions based on the science and data, free from political influence," White House spokesman Chris Meagher said in a statement. "That is what they did in this case and that is what we believe they should continue to do."

This account of the administration's surprise mask reversal is based on interviews with more than 15 senior administration officials, outside advisers and health experts, some of whom requested anonymity to candidly discuss internal policy deliberations.

Despite White House chief of staff Ron Klain and Jeff Zients, the administration's COVID coordinator, regularly quizzing health officials on when vaccinated Americans could safely take off their masks, the CDC did not inform the White House about the updated guidance until late Wednesday - leaving some senior officials frustrated at how abruptly they were informed.

Zients was told of the impending policy by Walensky around 6 p.m. that night, according to a CDC official, while aides said others in the West Wing weren't notified until around 9 p.m. Biden himself was not briefed until Thursday morning, officials said.

And when White House officials asked what they felt were basic questions - such as what the new guidance meant for businesses and children under 12 not yet eligible for vaccines - they felt CDC officials did not have sufficient answers. They also worried that the mixed messages over the course of the week would leave many Americans confused and key questions unanswered.

"You want talking points. You want to brief people," one person in contact with the White House said. "It's the right decision wrongly handled."

Officials said the administration's hands-off policy toward the CDC means that major shifts and decisions won't be communicated with the deftness that Americans might want. Still, several senior administration officials, outside advisers and health experts said the communication around the new guidance was fumbled.

"I think the biggest problem here is that this feels like an abrupt 180 to people. In reality, Dr. Walensky and Dr. Fauci have both explained the scientific reasoning underlying this," said Angela Rasmussen, research scientist and virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, referring to national infectious-diseases director Anthony Fauci. "But I think the fact that it was somewhat of a surprise underscores the need for transparency in formulating guidance and policy. In a pandemic, we are all stakeholders. If the public feels blindsided by these announcements, it suggests to me that they should have more opportunity to understand the basis for these decisions."

CDC officials defended Walensky's public statements throughout the week. Though her decision was made Monday night, "it could still have been derailed at any moment by a piece of evidence or concern from scientists on the team," one official said.

"They weren't going to release something until they were sure," the official said. In the meantime, "they have to defend the current guidance until we know we have new guidance."

The official added there were some within the agency who cautioned waiting, but she did not want to wait: "She had gotten to the point where she had seen enough science: the vaccines were working."

On Wednesday, two days after making her decision and the day after her combative hearing, Walensky was dropping hints about the upcoming change in posture.

Around 9:30 p.m. Wednesday, after telling Zients of her decision, CNN's Chris Cuomo confronted her with criticism that her agency was moving too slowly. "We're looking forward to updating it again very soon," Walensky promised.

On CNBC at 7 p.m. that evening, she was asked whether "there might be something" the next day. "We are looking forward to updating our guidance very soon," she said. "Stay tuned."

The CDC officially put out the news at 2 p.m. Thursday during a regular COVID briefing. Around 2:40 p.m. White House staff received an email informing staff that fully vaccinated employees didn't need to keep wearing masks.

As administration officials digested the updated guidance on Thursday, they scrambled to prepare for Biden to address the nation and announce the changes. The Rose Garden was prepared as the venue for the address by midmorning. Reporters learned that Biden would be speaking about two hours before he was set to give his address.

The president, in a jubilant mood, declared the significant milestone in an afternoon speech at the Rose Garden, where he appeared without a mask with Vice President Kamala Harris.

"We will rebuild our economy, reclaim our lives and get back to normal," he said. "We'll laugh again. We'll know joy again. We'll smile again - and now, see one another's smile."

But the excitement belied anxiety pulsing through parts of the administration about the updated guidance. Despite all the progress the country has made in vaccinating large swaths of the population - about 36% of Americans are fully vaccinated - administration officials privately noted there is still an average of roughly 35,000 new COVID cases and more than 600 deaths each day. Now, as people remove their masks indoors and relax social distancing guidelines, there are worries cases could spike in areas where many still remain unvaccinated.

Several experts stressed that the new guidance is supported by a growing body of scientific evidence. The three vaccines authorized for use in the United States - Pfizer-BioNTech, Moderna and Johnson & Johnson - are all highly effective against the variants circulating in the United States and protect almost entirely against severe illness and death.

Henry Walke, a CDC medical officer who has been leading the agency's response since July, said the steady decline in coronavirus cases, improving vaccination coverage and the increasing number of studies demonstrating real-world effectiveness in reducing secondary transmission to others, protection against asymptomatic infection and new virus variants "led to overwhelming agreement within the response that this is the right move at the right time."

In particular, a study in JAMA earlier this month that found evidence that a second dose of the Pfizer vaccine protects against all infections - even asymptomatic infection - helped convince agency scientists that it was time to change the guidance.

Still, across the administration, officials conceded the rollout of the updated guidance was shaky and the country would have benefited from a more carefully prepared announcement.

By Friday, businesses were struggling to adapt with signs requiring masks still hanging on their windows. Parents wondered what it meant for their children who were not yet eligible for a vaccine. And even the White House was still figuring out if more staff could start working in the West Wing and executive office buildings.

"The communications strategies on all of this are terrible," said a Biden ally who is in close contact with the administration. "You want to give people a little bit of notice."

Administration officials were also wrestling with questions about equity, a major focus of the Biden White House, because of disparities in vaccination rates between White communities and communities of color. Black and Latino communities have been hit particularly hard by the virus, and they have received a smaller share of vaccinations in the country to date. Health officials have made progress in closing these gaps, but they acknowledge there is still much work to be done.

Meanwhile, Republicans have seized on the timing of the updated guidance, suggesting the withering criticism at Tuesday's hearing expedited this week's rollout.

"The science on this has been the science for months," one Senate Republican aide said. "Nothing changed this week. Politically, however, a lot was new. Given the timeline, and the lack of new scientific evidence, it's really difficult to believe that Tuesday's hearing had zero impact on the White House's decision-making. The whole thing reeks of politics."

Despite Walensky's public posturing throughout the week, the CDC began working to change the guidance after Walensky had made the decision on Monday night.

After the CDC released its initial guidance last month that fully vaccinated people did not need masks outdoors, Walensky tapped Walke and his two main deputies to assess the next phase of the mask guidance. Walke said he and other scientists at the CDC had daily discussions with Walensky for weeks about what fully vaccinated people can do next.

"We were talking about this move over the weekend, and then she made a decision," Walke said in an interview Friday, adding that neither the White House nor the Department of Health and Human Services were involved in discussions about the scientific merits. The CDC is now reviewing travel, summer camp and school guidance for expected updates, officials said.

Although several experts said the latest guidance was justified, they cautioned that the CDC still needs to bring more of the public on board by explaining its decision. They also said the agency needed to provide more guidance in coming days and weeks to answer the many questions Americans still have about how to protect themselves with the majority of the country remains unvaccinated.

Those include how businesses should think about their mask policies, what parents with children under 12 should do, its guidance for summer camps and whether there needs to be a national discussion about vaccine passports and proof of vaccination.

"Right now we are immersed in a great debate about immune passports or some way of determining if people are protected," said Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota who was part of Biden's coronavirus task force during the transition. "In some ways you can say it's been hidden behind the mask and now it's out there in a very public way. That conversation had to come here. It's almost as if it should've been part of this discussion."