None is known to be infected with the disease, Centers for Disease Control and Prevention spokesman Tom Skinner said Tuesday.
The returning workers are clinicians for Partners in Health
, a Boston-based aid group. They all had contact with a colleague who's been diagnosed with the disease and is being treated at the National Institutes of Health in Bethesda, Maryland.
The heath care worker with Ebola was in critical condition Monday, the NIH said. That changed from the NIH saying on Friday that the patient was in serious condition. Details about the patient's identity weren't released.
While in West Africa, the workers "came to the aid of their ailing colleague," according to a Partners in Health statement.
As the CDC investigates who else might have had contact with the Ebola patient, more workers might be flown back to the United States, according to Skinner.
The workers are being housed near hospitals that specialize in treating Ebola patients, and if they show signs of the disease, they'll be admitted as patients -- as was the case with one worker in Nebraska.
The person developed symptoms Sunday evening and "out of an abundance of caution" was taken to the biocontainment unit at the University of Nebraska Medical Center in Omaha.
"At this point, this person has not tested positive for the Ebola virus," Phil Smith, M.D., medical director of the unit, said in a statement Monday. "However, because of a change in symptoms, we decided the most prudent course of action was to bring the individual to the Biocontainment Unit, where we can better monitor symptoms and safely perform testing. However, some of the symptoms which prompted the move to the Biocontainment Unit have resolved this morning."
Coming to Nebraska, Georgia, Maryland
The first of these workers arrived Friday night in Atlanta showing possible signs of Ebola, but testing over the weekend came back negative, according to a government official who did not want to be named.
The worker is being isolated in housing near Emory University Hospital for 21 days.
"Twice a day, we'll have visual monitoring, either face to face or we'll Skype with them, or do FaceTime," said Nancy Nydam, spokeswoman for the Georgia Department of Public Health. "And if it's Skype or FaceTime, they'll have to be in a place where we can clearly identify that they are where they're supposed to be."
The next set of aid workers returned back to the United States on Saturday evening and went into isolation in housing on the campus of the University of Nebraska Medical Center, according to spokesman Taylor Wilson. As in Atlanta, these workers have voluntarily agreed not to leave their housing.
"They will be monitored so they'll stay there," Wilson said.
Another set of three workers arrived Sunday morning at housing near the National Institutes of Health. These workers will have more freedom to move around than the ones in Georgia and Nebraska.
According to Christopher Garrett, a spokesman for the Maryland Department of Health, his state will follow the CDC's guidelines
, which forbid monitored individuals from getting on public transportation but allow them to go outside as long as they stay at least 3 feet away from other people.
More workers arrived Monday morning in Atlanta and were isolated in housing near Emory.
The patient now at NIH is the second with Ebola admitted to the NIH hospital.
A deadly and stubborn epidemic
Nina Pham, a nurse at Texas Health Presbyterian Hospital Dallas, was admitted to NIH in October after she contracted the disease while treating Liberian national Thomas Eric Duncan. Pham recovered and was released free of disease. Duncan died.
Emory, the NIH and Nebraska are three of only four hospitals in the United States that have biocontainment units to deal with a highly infectious disease such as Ebola.
More than 10,000 people have died in a West African epidemic of Ebola that dates back to December 2013, according to the World Health Organization. Almost all of the deaths have been in Guinea, Liberia and Sierra Leone.
Ebola is spread by direct contact with the bodily fluids of an infected person.