Around a dozen U.S. aid workers were evacuated from Sierra Leone following fears of exposure to the Ebola virus.
They had been working at a clinic run by the U.S. charity Partners in Health in Port Loko district of the north of the country.
An American clinician back from Sierra Leone had tested positive and is being treated at a hospital near Washington DC. His condition is said to be serious.
Yet another aid worker who was flown in on March 13 after developing symptoms had tested negative twice but was being treated.
Investigators fear the clinician may have exposed others to the virus.