NEW YORK (AP) — Four more American aid workers arrived back in the United States on Tuesday from West Africa to be monitored for Ebola, health officials said.

The latest arrivals bring to 15 the number of aid workers who have returned from Sierra Leone since Friday. None of them have been diagnosed with Ebola, but they will be isolated and monitored during the next three weeks for signs of the disease.

Officials have released few details, citing patient privacy. But all are connected to — or had direct physical contact with — another American who came down with Ebola last week in Africa. The unidentified man is in critical condition at a National Institutes of Health hospital in Bethesda, Maryland.

The other aid workers are staying near the Maryland hospital or hospitals in Atlanta and Omaha with special isolation units — in case they become sick, according to officials with the Centers for Disease Control and Prevention.

The sick American works with Partners in Health, a Boston-based international aid organization that has been treating patients in Liberia and Sierra Leone since November. Organization officials did not respond to calls and emails from The Associated Press on Monday or Tuesday. In a statement Tuesday, co-founder Dr. Paul Farmer said the group is working with the CDC and others to investigate how the man was infected. The Ebola virus is spread through contact with a sick patient’s blood or bodily fluids.

The World Health Organization estimates the virus has killed more than 10,000 people, mostly in Guinea, Liberia and Sierra Leone. While deaths have slowed dramatically in recent months, the virus appears stubbornly entrenched in parts of Guinea and Sierra Leone.

Since last summer, several U.S. aid workers have gotten Ebola and have been flown back to the U.S. for care.