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.

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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. A few other individuals have returned for monitoring; none got sick.

The aid workers who have returned since Friday are by far the largest group. And more may be on their way later this week, CDC spokeswoman Kathy Harben said Tuesday.

The sick American worker was working at a government hospital in Port Loko in western Sierra Leone. But medical superintendent Dr. Peter George said the hospital hasn’t had any Ebola cases for quite some time. He said it’s likely the man also worked at a nearby Ebola treatment center, which George also manages. George said a Partners in Health colleague told him the American passed out while at the hospital.

“At that time, nobody knew it was Ebola, so they assisted him,” he said.

Everyone who was working on the ward with the man is now being monitored, George said. He said that many of the foreigners working in Port Loko, including some of the Americans, live at the same tent camp. He did not know if the sick health worker was living there.

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Associated Press writer Sarah DiLorenzo in Dakar, Senegal, contributed to this report.