US health 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 second person infected in the US, Amber Vinson, 29, fell ill on Tuesday.
Both she and nurse Nina Pham, 26, had treated Liberian Thomas Eric Duncan, who died a week ago in Dallas.
A nurses' union has said those treating Duncan were not given full protection and had parts of their skin exposed
More than 70 healthcare workers who may have come in contact with him at the hospital are being monitored for symptoms, the hospital's director has said.
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 Wednesday, the US Centers for Disease Control and Prevention (CDC) said it wanted to interview the passengers on Frontier Airlines flight 1143 from Cleveland, Ohio, to Dallas, Texas on 13 October.
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 Ms Vinson and Ms Pham treated Duncan early in his stay at Texas Health Presbyterian Hospital in Dallas when he had "extensive production of body fluids", said CDC director Tom Frieden.
A national nurse union told reporters on Wednesday the health workers treating Duncan had not been properly protected and called for all health workers treating Ebola patients to receive full protective suits and training from hospitals.
Union director RoseAnn DeMoro said staff treated him for days without the necessary protective gear, and hazardous waste was allowed to pile up to the ceiling.
The CDC has appointed a "site manager" in Dallas to standardise the protective equipment and supervise the method of putting it off and on.
Ms Vinson flew to Cleveland on 10 October, even though she was being monitored for signs of Ebola and therefore should not have flown on a commercial aeroplane, Dr Frieden said.
When Ms Vinson returned from Ohio, 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.
On the morning of 14 October, Ms Vinson came down with a fever and was isolated within 90 minutes. Her diagnosis was announced early on Wednesday.
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.
Mr Duncan, who was the first person to be diagnosed in the US with Ebola, started showing symptoms of the disease just days after he arrived in Texas from Liberia, where he contracted it.
An initial set of 48 people who were in contact with him before he was admitted to hospital are nearing the end of the window in which they could develop an Ebola infection.
Ebola patients treated outside West Africa*
*In all but three cases the patient was infected with Ebola while in West Africa. Infection outside Africa has been restricted to health workers in Madrid and in Dallas. DR Congo has also reported a separate outbreak of an unrelated strain of Ebola.
- Avoid direct contact with sick patients as the virus is spread through contaminated body fluids
- Wear goggles to protect eyes
- Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated
- People who recover from Ebola should abstain from sex or use condoms for three months
The World Health Organization (WHO) says 4,447 people have died from the outbreak, mainly in West Africa.
Sierra Leone, Liberia and Guinea have been hardest hit by the outbreak, which began in December 2013 but was confirmed in March.
The White House has announced President Barack Obama will convene a meeting of his cabinet over the US response to Ebola later on Wednesday, cancelling a political trip.
Health officials have repeatedly warned more Ebola cases could be diagnosed in the US.
In other developments:
- Liberia's transport minister has gone into quarantine after her driver died from Ebola
- Ghana is to lodge a complaint with authorities in the Czech Republic after a video showed a Ghanaian student not infected with Ebola being carted around an airport on a trolley by people in protective gear - while his face is covered with a black plastic bag
- The World Health Organization (WHO) warns the infection rate could reach 5,000 to 10,000 new cases a week in two months if efforts are not stepped up
- The WHO says it will declare the end of the outbreak in Senegal at the end of this week (17 October) and in Nigeria next week (20 October), if no new cases are detected before then.
- France has joined the US, Canada and the UK in instituting Ebola screening at airports