A Texas health care worker who treated Ebola victim Thomas Duncan before his death has tested positive for the virus, 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.
Mr Duncan tested positive in Dallas on 30 September, 10 days after arriving on a flight from Monrovia via Brussels.
He became 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 in a preliminary test, came into contact with Mr Duncan when he first showed possible symptoms or when he was admitted to hospital.
He 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."
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.