A Texan health worker who treated Ebola victim Thomas Eric Duncan before he died is also infected with the virus, according to a preliminary test.
The unnamed woman, who is in a stable condition in an isolation ward, wore full protective gear while treating Duncan, officials in Dallas say.
If confirmed, this is the first known transmission of Ebola on US soil, the BBC reports.
A top federal health official said there had been a clear breach of safety protocol and other cases could follow.
Centers for Disease Control and Prevention (CDC) head Dr Tom Frieden said 48 people who may have had contact with Duncan were being monitored for symptoms.
He said a complete investigation would be conducted into how the infection had occurred at Texas Health Presbyterian Hospital.
Duncan, who caught the virus in his native Liberia, died on Wednesday.
The current Ebola outbreak, concentrated in Liberia, Guinea and Sierra Leone, has resulted in more than 8300 confirmed and suspected cases, and at least 4033 deaths.
No details of the health worker's identity or position at the hospital were given, in accordance with family wishes.
Dr Daniel Varga, of the Texas Health Resource, said she had worn a gown, gloves, mask and shield when providing care to Duncan during his second and final hospital admission.
Dr Tom Frieden said there had been a clear breach of protocol.
"I think the fact that we don't know of a breach in protocol is concerning because clearly there was a breach in protocol," he told American broadcaster CBS.
The CDC investigation would focus on possible breaches made during the "high-risk procedures" of dialysis and respiratory intubation, Dr Frieden said.
Tests on the infected health worker would be completed on Sunday.
United States President Barack Obama called for urgent action to ensure the US medical system could properly handle patients with Ebola.
Mr Obama said federal authorities needed to take additional steps to make sure correct protocols were followed after the apparent breach in Texas.