In the end, 61 Americans remain on board, officials said.

Asked why American officials began evacuating passengers without knowing their test results, Dr. William Walters, managing director of operational medicine at the State Department, told reporters on Monday that it was “unpredictable” when the results would come back.

The Americans were put on about 15 buses to the airport. Once it became clear that some had the coronavirus, the State Department said, the infected passengers “were moved in the most expeditious and safe manner to a specialized containment area” in the rear of the planes.

“In the isolation area, they posed no additional risk,” Dr. Walters said.

After landing, the 14 infected evacuees went to hospitals for monitoring and treatment. Most ended up being flown to Omaha for medical treatment by experts at the University of Nebraska. Another five returning passengers were also put into isolation because they had developed fevers.

The remaining American passengers stayed at Travis Air Force Base in California or Lackland Air Force Base in Texas, where they had landed and where they will remain under quarantine for an additional 14 days.

When one of the planes landed in California, a line of officials from the military, the Centers for Disease Control and Prevention, and the Department of Homeland Security greeted passengers with banners that read “Welcome home.”

After being ushered through an isolation tent, they were assigned to apartments on the base.

“They have flown in specialists from across the country,” said one evacuee, Sarah Arana, 52, a medical social worker from Paso Robles, Calif. “It’s a phenomenal amount of resources. I’m kind of blown away.”

Epidemiologists said American officials had made a difficult decision in allowing infected passengers onboard the charter flights.