There have been at least 634 infections and two deaths from the Diamond Princess, which Japanese officials kept in isolation for two weeks at a port in Yokohama. That effort at a quarantine contributed to the virus’s rapid spread among passengers. The cluster from the ship is the largest concentration of coronavirus cases outside China, warranting its own category in data compiled by the World Health Organization.

American officials began a complex evacuation procedure on Sunday night for 328 passengers aboard the Diamond Princess. All had been examined by American medical experts and showed no symptoms of the coronavirus, Dr. William Walters, managing director of operational medicine at the State Department, and Dr. Robert Kadlec, assistant secretary for preparedness and response at the Department of Health and Human Services, said on Monday during a conference call with reporters.

But as those passengers were bused to Haneda Airport in Tokyo early Monday morning, Japanese officials told American counterparts that laboratory tests for 14 passengers had come back positive, Dr. Walters said. The tests had been conducted two to three days earlier, but American officials, believing the timing of the results would be “unpredictable” because of the volume of testing being done in Japan, began the evacuation without having all results in hand.

American passengers who had already tested positive or who had displayed symptoms had been sent to hospitals in Japan, Dr. Walters said.

After they learned that 14 passengers had tested positive, American officials decided that the entire group set to leave Japan should be treated according to protocols the officials had developed for evacuees, Dr. Walters said. That meant continuing to transport those who had tested positive but putting them in isolation — behind sheets of plastic about 10-feet tall — at the rear of the two planes flying them back to the United States.

Dr. Walters said on Monday that he and Dr. Kadlec reviewed the possible options after learning of the test results.