An American health care worker who contracted Ebola while volunteering in a Sierra Leone treatment unit arrived safely at the National Institutes of Health's hospital in Maryland, officials said Friday.

Doctors evaluated the patient, and the person is in serious condition after being flown in isolation from Sierra Leone on a chartered plane and admitted at 4:44 a.m., NIH officials said in a statement. The patient's name, age and gender were not released.

A statement from Centers for Disease Control and Prevention (CDC) Friday said that they were also investigating other potential exposures of American citizens to Ebola in West Africa. According to the statement, one of the citizens had potential exposure to the individual being treated at NIH and is being transported via charter to the Atlanta area to be close to Emory University Hospital. The individual has not shown any symptoms of Ebola or received a diagnosis. The CDC said the individual will voluntarily self-isolate and be under direct active monitoring for the 21-day incubation period. 

The patient at NIH is the 11th person with Ebola to be treated in the U.S. and the second admitted to the NIH Clinical Center. It has one of the few specialized isolation units nationwide set up to treat Ebola patients.

The center's Special Clinical Studies Unit is staffed by specialists in infectious disease and critical care and is designed to prevent the spread of highly contagious viruses, including Ebola. Previously, an American nurse was treated there after she contracted Ebola while caring for a patient in a Dallas hospital. The nurse, Nina Pham, survived and is Ebola-free.

The World Health Organization estimated Thursday that the virus has killed more than 10,000 people, mostly in the West African nations of Guinea, Liberia and Sierra Leone. The current outbreak is the largest ever for the disease. While deaths have slowed dramatically in recent months, the virus appears stubbornly entrenched in parts of Guinea and Sierra Leone.

The Associated Press contributed to this report.