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WASHINGTON (AP) — A top federal health official says the Ebola diagnosis in a health care worker who treated Thomas Eric Duncan at a Texas hospital shows there was a clear breach of safety protocol.

Dr. Tom Frieden, head of the Centers for Disease Control and Prevention, says the worker had treated Duncan multiple times after the Liberian man was diagnosed.

Frieden tells CBS' "Face the Nation" that all those who treated Duncan are now considered to be potentially exposed. Frieden couldn't give an exact number.

Health care workers treating Duncan were to follow CDC protocol that included wearing protective gear.

Among the things CDC will investigate is how the workers took off that gear — because removing it incorrectly can lead to a contamination.

Duncan died of the disease last Wednesday.