(Reuters) - An American being treated for Ebola was in critical condition Monday, the U.S. National Institutes of Health in Maryland said.

The healthcare worker was evacuated from West Africa and arrived at the NIH clinical center on Friday. The clinician, who has not been identified, was working with Partners In Health at the medical aid group's Sierra Leone Ebola center.

Partners in Health said in a statement that 10 of its clinicians in Sierra Leone came to the aid of their ailing colleague. They were all subsequently identified as contacts of the patient being treated at NIH.

The 10 individuals were flown to the United States via non-commercial aircraft to be monitored at designated Ebola treatment facilities to give them access to rapid testing and, if necessary, treatment.

Four are being monitored by doctors at Nebraska Medicine; one of them developed symptoms on Sunday evening and was moved into the Nebraska Medicine biocontainment facility, Dr. Phil Smith, medical director of the biocontainment unit, said in a statement. However, Smith said some of those symptoms have since resolved.

"At this point, this person has not tested positive for the Ebola virus," Smith said.

The hospital said a fifth person from the original exposure group in Sierra Leone is being transferred to the Nebraska facility for monitoring later on Monday.

The healthcare workers will voluntarily isolate themselves during the remainder of the 21-day Ebola incubation period, Partners in Health said.

Tom Skinner, a spokesman for the U.S. Centers for Disease Control and Prevention, said the agency is continuing to investigate the circumstances around the 10 exposures.