The medical charity Partners in Health said the patient is a clinician who worked in the West African country, which along with Liberia and Guinea is struggling to emerge from the worst Ebola outbreak in history.
The patient was airlifted from Sierra Leone and was admitted to the National Institutes of Health Clinical Center on Friday.
"NIH physicians have changed the status of the patient with Ebola virus disease being treated at the NIH Clinical Center from serious to critical condition," a statement said.
Ten other healthcare workers "who came to the aid of their ailing colleague were subsequently identified as contacts of the evacuated clinician," said Partners in Health.
They are being transported to the United States and are being monitored for signs of illness.