Amid the troubling news that a nurse traveled on a crowded commercial flight a day before she was diagnosed with the Ebola virus, public health officials testifying on Capitol Hill defended their opposition to banning all travelers from Ebola-stricken countries in West Africa.

Yet while they argued for allowing travel to continue, insisting that such a ban would make it harder to check passengers at airports on both sides of the Atlantic Ocean, they also have not completely ruled out a ban.

“Right now we are able to track everyone who comes in,” Thomas Frieden, director of the Centers for Disease Control and Prevention, said during a congressional hearing Thursday. But he added: “We will consider any options to better protect Americans.”

This hearing came a day after a stream of bad news, with the announcement early Wednesday that another health-care worker was diagnosed with Ebola, followed closely by the revelation that she had a slight fever before boarding a Frontier Airlines flight from Ohio to Texas. And it was confirmed later in the day that she had told the CDC about her temperature but was still allowed to fly.

“It’s not a drill,” Rep. Fred Upton (R-Mich.), chairman of the House’s Energy and Commerce Committee and a member of the subcommittee holding the hearing, said during his opening remarks. “People’s lives are at stake, and the response so far has been unacceptable.”

Upton was particularly critical of the fact that the nurse flew across the country on Monday, despite her slightly elevated temperature and even though another nurse had been diagnosed with Ebola just a day before her flight.

Frieden said that while the nurse had contacted the CDC before traveling, he was not clear on exactly what the conversation entailed, but he believed she had reported no symptoms to the agency.

He said the CDC is dedicated to protecting Americans, but he also stressed again that the risk cannot be fully eliminated in the United States until the outbreak is staunched in West Africa.

Frieden and a hospital official said that it was still unclear how the two nurses — Nina Pham and Amber Vinson — were infected while caring for Thomas Duncan, a Liberian man diagnosed with Ebola in Texas last month.

“While we do not yet know exactly how these transmissions occurred, they demonstrate the need to strengthen the procedures for infection-control protocols which allowed for exposure to the virus,” Frieden wrote in his prepared remarks.

He later said that an ongoing investigation has identified possible causes of the infection.

Potential treatments are being tested in clinical trials and are being used in experimental ways to treat Ebola patients, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, said in his opening remarks. “We have been working on an Ebola vaccine for a number of years.”

However, it is unclear whether the possible vaccines being discussed will be effective or safe, he said, which is why they require trials and testing.

The Food and Drug Administration has fast-tracked the two vaccines currently being developed, said Luciana Borio, assistant commissioner for counterterrorism policy at the agency. But she stressed the importance of clinical trials to make sure the vaccines actually work.

“There is tremendous hope that some of these products will help patients,” Borio said. “But it is also possible some may hurt patients, or others will have little or no effect.”

Still, she said that all three Ebola patients in the United States so far have been treated with at least one experimental drug.

As the hearing was going on, it was announced that Pham, the first nurse infected with Ebola, would be transferred to a National Institutes of Health clinical center in Bethesda. Vinson was transferred to Emory University Hospital on Wednesday evening, which means that the hospital that treated the first three Ebola diagnoses in the United States no longer has any Ebola patients.

At least 50 health-care workers at the Dallas hospital, which has become the epicenter of Ebola in the United States, are being monitored for possible exposure, Frieden said.

“With many of the medical professionals who would normally staff the intensive care unit sidelined for continuous monitoring, it is in the best interest of Nina, hospital employees, nurses, physicians and the community to give the hospital an opportunity to prepare for whatever comes next,” the hospital said in a statement Thursday.

In a statement issued by the hospital, Pham thanked people for “the  outpouring of love and support” she said she has received since her diagnosis.

“I’m doing really well thanks to this team, which is the best in the world,” the statement said. “I believe in my talented co-workers.”

The hospital also apologized for its initial mishandling of Duncan, who flew from Liberia to Texas and was diagnosed with Ebola a little more than a week later.

Daniel Varga, chief clinical officer of the health-care system that oversees Texas Health Presbyterian Hospital in Dallas, said in a prepared statement for the hearing that the hospital “made mistakes” when Duncan initially sought treatment but was released.

“We did not correctly diagnose his symptoms as those of Ebola,” Varga said to the committee. “We are deeply sorry.”

The hearing repeatedly returned to the idea of a travel ban, which has been discussed by politicians since the days after Duncan’s diagnosis. House Speaker John A. Boehner (R-Ohio) and other lawmakers are calling for a ban on travelers from Guinea, Liberia and Sierra Leone, the three countries ravaged by the outbreak. About 150 people arrive in the United States from these countries each day, the majority of them flying into five airports in or around New York, Washington, Chicago and Atlanta.

Enhanced screening meant to check these travelers began last week at New York’s John F. Kennedy International Airport and expanded Thursday to four other airports, including Dulles International Airport outside Washington, D.C.

Still, Frieden said that authorities know who is traveling from those countries right now. If a travel ban is enacted, he said, people potentially infected could travel by land to another country before flying to the United States, which means it may not be clear they were in a country with Ebola and they would not be checked for fever while leaving or arriving.

“We are dealing with something now that we know what we’re dealing with,” Fauci said.

After a campaign event in St. Paul, Sen. Al Franken (D-Minne.) joined the growing chorus of lawmakers who said some form of ban on travel should be implemented if the proper safeguards can be employed to allow for aid workers and other officials.

“We have to consider that,” Franken told reporters.

In response to mounting concerns over Ebola, President Obama canceled a campaign trip and convened a Cabinet meeting Wednesday. On Thursday, he also canceled additional trips, and the White House said he would remain there to follow up on the government’s response to the virus.

Obama signed an executive order Thursday allowing for National Guard troops and reservists to be called upon, if necessary, to help with the response to Ebola in West Africa.

Paul Kane contributed to this report.

[This post was updated throughout the hearing. Last update: 5:21 p.m.]