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.
A top federal health official said there had been a clear breach of safety protocol and other cases could follow.
Dr Tom Frieden, head of the Centers for Disease Control and Prevention (CDC), 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 8,300 confirmed and suspected cases, and at least 4,033 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 US broadcaster CBS.
The CDC investigation, he told reporters, would focus on possible breaches made during two "high-risk procedures", dialysis and respiratory intubation.
Tests on the infected health worker, he confirmed, would be completed on Sunday.
Mike Rawlings, the mayor of Dallas, said steps had been taken to decontaminate common areas in the patient's apartment complex as well as the patient's car.
Police have been guarding the apartment complex, where a barrel labelled "biohazard" could be seen on a lawn outside.
The health worker reported a low-grade fever on Friday night and was isolated and referred for testing, officials said. Preliminary test results were received late on Saturday.
Judge Clay Jenkins, chief executive of Dallas County, said the infected health worker was a "heroic person".
"This is obviously bad news, it is not news that should bring about panic," he added.
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.
Protective Ebola suit×
The cap forms part of a protective hood covering the head and neck. It offers medical workers an added layer of protection, ensuring that they cannot touch any part of their face whilst in the treatment centre.
Goggles, or eye visors, are used to provide cover to the eyes, protecting them from splashes. The goggles are sprayed with an anti-fogging solution before being worn. On October 21, the US Centers for Disease Control and Prevention (CDC) announced stringent new guidelines for healthcare personnel who may be dealing with Ebola patients. In the new guidelines, health workers are advised to use a single use disposable full face shield as goggles may not provide complete skin coverage.
Covers the mouth to protect from sprays of blood or body fluids from patients. When wearing a respirator, the medical worker must tear this outer mask to allow the respirator through.
A respirator is worn to protect the wearer from a patient's coughs. According to guidelines from the medical charity Medecins Sans Frontieres (MSF), the respirator should be put on second, right after donning the overalls.
A surgical scrub suit, durable hospital clothing that absorbs liquid and is easily cleaned, is worn as a baselayer underneath the overalls. It is normally tucked into rubber boots to ensure no skin is exposed.
The overalls are placed on top of the scrubs. These suits are similar to hazardous material (hazmat) suits worn in toxic environments. The team member supervising the process should check that the equipment is not damaged.
A minimum two sets of gloves are required, covering the suit cuff. When putting on the gloves, care must be taken to ensure that no skin is exposed and that they are worn in such a way that any fluid on the sleeve will run off the suit and glove. Medical workers must change gloves between patients, performing thorough hand hygiene before donning a new pair. Heavy duty gloves are used whenever workers need to handle infectious waste.
A waterproof apron is placed on top of the overalls as a final layer of protective clothing.
Ebola health workers typically wear rubber boots, with the scrubs tucked into the footwear. If boots are unavailable, workers must wear closed, puncture and fluid-resistant shoes.
But despite telling medical staff he had been in Liberia, he was sent home with painkillers and antibiotics.
Duncan was later put into an isolation unit at the hospital but died despite being given an experimental drug.
A nurse in Spain contracted Ebola while caring for patients who came from West Africa. Hers was the first case of transmission of the virus outside West Africa.
A Spanish health ministry official said on Sunday that there were "high hopes" that the condition of the nurse, Teresa Romero, was coming under control.
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 contained 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.
Ebola deaths: Confirmed, probable and suspected
Note: figures have occasionally been revised down as suspected or probable cases are found to be unrelated to Ebola. They do not include one death in the US recorded on 8 October.
- Avoid direct contact with sick patients
- 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
Have you been affected by the Ebola outbreak? Do you think enough is being done where you live to fight the disease? Tell us your story. You can email firstname.lastname@example.org.