A Dallas health care worker who provided care for the Ebola patient who died there last week tested positive for the deadly virus — and sent health officials scrambling Sunday to find the “breach in protocol” that resulted in her infection.

The woman was among caregivers for Thomas Eric Duncan, who died Wednesday at Texas Health Presbyterian Hospital. If the state’s test is confirmed by the Centers for Disease Control and Prevention, it would be the first known Ebola case transmitted in the U.S.

CDC chief Thomas Frieden said Sunday his agency will investigate how a worker in full protective gear contracted the virus.

“At some point there was a breach in protocol,” Frieden said. “That breach in protocol resulted in this infection.”