He and others learned of the labeling mistake from news reports before they were told by staff members on the base, he said, angering some who took their frustration out on C.D.C. officials during a meeting on the base on Monday night.

The episode began last week when three people who had been quarantined at the San Diego base showed symptoms of illness and were taken to the hospital not long after they arrived in the United States.

A miscommunication between C.D.C. officials led the agency to tell the hospital that all the patients had tested negative even though three of the samples had not been tested, Dr. Christopher R. Braden, a medical epidemiologist with the C.D.C., said at a news conference on Tuesday evening. Thinking that the three were clear of the virus, the hospital sent the patients back to the base to complete the 14-day quarantine period with scores of others.

The error was discovered as the three people, wearing masks, were riding in a van back to the base on Sunday. Their specimens, officials realized, had never been analyzed.

“Of course, as luck would have it, there was one of those tests that came back positive,” Dr. Braden said.

Rather than return the three to the hospital, health officials decided to proceed to the base, telling the patients to isolate themselves in their rooms, officials said.

The test results came back on Monday morning. One of the three people — identified only by her gender — was positive for the virus and was rushed back to the hospital. She was being treated there and was doing well, with just a minor cough, Dr. Braden said.