A Texas health care worker who treated US 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, but after going to hospital and telling medical staff he had been in Liberia, he was sent home with antibiotics.
He was later put into an isolation unit at Texas Health Presbyterian Hospital in Dallas but died despite being given an experimental drug.
It is not clear at which point the health worker, who has tested positive in a preliminary test, came into contact with Mr Duncan.
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.