Testing performed on the body of a woman who died in Sierra Leone has come back positive for the Ebola virus, officials say.
If confirmed, it would represent a setback for efforts to eradicate the disease in the West African nations, which announced less than a week ago the release from hospital of the last known Ebola patient.
That release should have marked the beginning of a 42-day countdown period that would have allowed Sierra Leone to be declared Ebola-free if no new cases were diagnosed.
Officials in the Kambia district in the north of Sierra Leon said they were trying to determine if the 67-year-old deceased woman died before or after the beginning of the countdown.
Teams of health workers had been dispatched there to do surveillance and trace people who may have come in contact with the woman, they say.
"We should not despair, as we have been expecting this," said O.B Sisay, director of Sierra Leone's National Ebola Response Center. "We need to stay focused and maintain our discipline."
In the worst Ebola outbreak in history that ravaged Guinea, Liberia and Sierra Leone, more than 11,300 people died, 4,000 of them in Sierra Leone.
The World Health Organization's Ebola countdown requirement for declaring a country Ebola-free is based on a time period twice the length of the virus disease's 21-day incubation period.
Two samples from the woman who died in Kambia tested positive, but officials said they were carrying out further tests in Makeni, the largest city in the Northern Province, and would also conduct tests in the nation's capital Freetown.
"We are particularly concerned because Kambia has gone 50 days without a confirmed Ebola case, suggesting the possibility of an error," said the country's chief medical officer Brima Kargbo.
The Kambia District lies along Sierra Leone's border with Guinea.