A preliminary test shows a Texas health worker who treated Ebola victim Thomas Duncan before he died is also infected with the virus, US officials say.
“We knew a second case could be a reality, and we’ve been preparing for this possibility,” said Dr David Lakey, commissioner of the Texas Department of State Health Services.
Mr Duncan, who caught the virus in his native Liberia, died at a Dallas hospital on Wednesday.
The health worker has not been named.
The current Ebola outbreak, concentrated in Liberia, Guinea and Sierra Leone, has resulted in more than 8,300 confirmed and suspected cases, and at least 4,033 deaths.
Sent home by hospital
Mr Duncan tested positive in Dallas on 30 September, 10 days after arriving on a flight from Monrovia via Brussels.
He had become ill a few days after arriving in the US, and went to the Texas Health Presbyterian Hospital in Dallas with a high fever.
But despite telling medical staff he had been in Liberia, he was sent home with painkillers and antibiotics.
Mr Duncan was later put into an isolation unit at the hospital but died despite being given an experimental drug.
It is not clear whether the health worker who tested positive came into contact with Mr Duncan when he first showed possible symptoms or when he was re-admitted to hospital.
The health worker reported a low-grade fever on Friday and was isolated and referred for testing, Texas health care services said in a statement.
“Health officials have interviewed the patient and are identifying any contacts or potential exposures,” it added.
“People who had contact with the health care worker after symptoms emerged will be monitored based on the nature of their interactions and the potential they were exposed to the virus.”
Some 50 people who had direct or indirect contact with Mr Duncan are reportedly already being monitored in case they develop symptoms.
More tests on the US health care worker are being carried out at the Centers for Disease Control and Prevention in Atlanta. If Ebola is confirmed, it would be the first time the infection is known to have spread within the US.
A nurse in Spain contracted the haemorrhagic fever while caring for patients who came from West Africa.
The arrival of Ebola in Texas prompted the US authorities to introduce screening of passengers from affected countries at airports, starting on Saturday at New York’s JFK.
Passengers from those countries will have their temperatures taken and have to answer a series of questions.
With the numbers of those affected continuing to rise in West Africa, the UN special envoy on Ebola says he hopes that the outbreak can be brought under control within three months.
David Nabarro told the BBC the number of Ebola cases was currently increasing exponentially, but greater awareness would help contain the virus.
“I think we’ve got much better community involvement [now] which leads me to believe that getting it under control within the next three months is a reasonable target,” he said.
Mr Nabarro described the accelerating increase of new cases as “quite frightening”.