The director of the Centers for Disease Control and Prevention this morning said he is “deeply concerned” about a second person in the United States testing positive for the deadly Ebola virus.

The patient, an unidentified health care worker at Texas Health Presbyterian Hospital, was among those who had treated Thomas Eric Duncan, the man visiting the Dallas area from Liberia who died Wednesday of Ebola. The worker is a woman, the Associated Press reported.

>>> Second US Ebola Patient Identified as Young Texas Nurse

“We’re deeply concerned about this new development with a preliminary positive [test for Ebola],” CDC Director Thomas Frieden said. “The fact that we don’t know about a breach in protocol is concerning because, clearly, there was a breach in protocol.”

>>> Update: CDC confirmed preliminary test results showing that the health worker has Ebola, the agency said in a statement released late this afternoon. She is being kept in isolation at the hospital and “one close contact … is being monitored,” CDC said. It said officials “remain confident that wider spread in the community can be prevented with proper public health measures.”

Since Duncan is believed to already have been infected when he arrived Sept. 20 in the U.S.,  confirmation of the preliminary diagnosis would make this the first known case of Ebola being contracted or transmitted here.

The hospital worker, who reported a fever Friday night, is listed in stable condition. The worker provided care for Duncan “on multiple occasions after his diagnosis” Sept. 28, wearing protective gear that included a gown, gloves, mask and shield.

Frieden, appearing on CBS’s “Face the Nation,” said CDC will pursue a detailed investigation.

“Even a single breach [of safety protocol] can result in contamination,” he said. “We have the ability to prevent the spread of Ebola by caring safely for patients.”

>>> Commentary: Solution to Ebola Isn’t Banning Travel

Dallas County Judge Clay Jenkins urged against overreaction at a press conference this morning.

“While this is obviously bad news, it is not news that should bring about panic,” he said.

Although patient confidentially prevented Frieden or Jenkins from revealing more personal  information, House Homeland Security Chairman Mike McCaul, R-Texas, said on “Face the Nation” that the new Ebola patient is a nurse and “most likely female.”

Preliminary test results from the Texas Department of State Health Services, received late last night, were to be reviewed by CDC in Atlanta, officials said.

Although CDC could not specify how many persons may have come into contact with Duncan, Frieden said at-risk individuals are being monitored closely and kept away from the public.

>>>Read More: Q&A: How Worried Should We Be About Ebola Spreading in U.S.?

  • Ken McIntyre, news director of The Daily Signal, contributed to this report.