The Centers for Disease Control and Prevention (CDC) officials are seeking 132 people who flew on a plane with a Texas nurse on the day before she came down with symptoms of Ebola.
The nurse, the second person to catch Ebola in the US, became ill on October 14.
Both she and nurse Nina Pham, 26, had treated Liberian Thomas Eric Duncan, who died on October 8, in Dallas.
Meanwhile, the UN’s Ebola mission chief says the world is falling behind in the race to contain the virus, which has killed more than 4,000 in West Africa.
On October 15, the CDC said it wanted to interview the passengers on Frontier Airlines flight 1143 from Cleveland, Ohio, to Dallas, Texas on October 13.
It said it was taking the measure “because of the proximity in time between the evening flight and first report of illness the following morning”.
Both the newly diagnosed nurse, who has yet to be identified, and Nina Pham treated Thomas Eric Duncan early in his stay at Texas Health Presbyterian Hospital in Dallas when he had “extensive production of body fluids”, CDC director Tom Frieden told reporters on Wednesday.
The second nurse flew to Cleveland on 10 October, even though she had had “extensive contact” with Thomas Eric Duncan and was being monitored for signs of Ebola and therefore should not have flown on a commercial aeroplane, Dr. Tom Frieden said.
Nina Pham subsequently became ill and was diagnosed with Ebola. When the second nurse returned from Ohio on Monday evening, she was not showing symptoms of the disease, the crew has told CDC investigators.
Health experts say people who are not showing symptoms are not contagious.
“We will from this moment forward ensure that no other individual who is being monitored for exposure undergoes travel in any way other than controlled movement,” Dr. Tom Frieden said, meaning, for example, in chartered flights or ambulances.
On the morning of October 14, the second nurse came down with a fever and was isolated within 90 minutes. Her diagnosis was announced early on October 15.
One of the ill women is to be transferred to Emory University hospital in Atlanta, which oversaw the recovery of two US aid workers who had caught the disease in Africa.