OMAHA, Neb. (AP) — One of the American aid workers who were exposed to Ebola in Africa is being treated in a Nebraska hospital’s isolation unit after developing symptoms.

Nebraska Medical Center spokesman Taylor Wilson says the individual developed symptoms Sunday evening and was hospitalized as a precaution. Wilson declined to describe the symptoms, but said they had been resolved Monday.

The hospitalized individual had not tested positive for Ebola Monday.

Three others continue being monitored, and another aid worker exposed to Ebola in Sierra Leone is expected to join them Monday afternoon.

The aid workers are part of a group that was evacuated after one of their colleagues contracted the deadly virus. They were all working with the Boston-based nonprofit Partners in Health.