DALLAS (AP) — A “breach of protocol” at the hospital where Ebola victim Thomas Eric Duncan was treated before his death led to the infection of a health care worker with the deadly virus, and other caregivers could potentially be exposed, federal health officials said Sunday.

The hospital worker, a woman who was not identified by officials, wore protective gear while treating the Liberian patient, and she has been unable to point to how the breach might have occurred, said Dr. Tom Frieden, head of the Centers for Disease Control and Prevention. Duncan was the first person in the U.S. diagnosed with Ebola.