Story highlights Eight have already returned and three more are due Monday

The returning workers are clinicians for Partners in Health, a Boston-based aid group

None of them is showing symptoms so far

(CNN) Eight American aid workers who were exposed to Ebola in Sierra Leone have been flown back to the United States where health authorities will watch them closely for signs of the disease, according to the Centers for Disease Control and Prevention.

Three more exposed workers are due to arrive back in the United States on Monday.

The returning workers are clinicians for Partners in Health, a Boston-based aid group. None of them is showing symptoms of Ebola, but 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.

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 Tom Skinner, a spokesman for the CDC.

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