A health care worker in Dallas appears to have become the first person to contract Ebola inside the United States. The worker at the Texas Health Presbyterian Hospital helped take care of Thomas Eric Duncan, who was the first person to die of Ebola in the country after he contracted the virus in his native Liberia. The health care worker tested positive for Ebola in a preliminary analysis at the state public health lab in Austin, which the Centers for Disease Control and Prevention must now confirm, according to a statement. What seems most surprising—and, yes, scary—about this transmission is that the worker was infected with the virus despite wearing full protective gear while in contact with Duncan, reports the Texas Tribune. That means the worker wore “a gown, gloves, mask and shield” while providing care, notes the Associated

“We are very concerned,” Texas Health Resources chief clinical officer Dan Varga told a news conference, according to Reuters. “We don’t have a full analysis of all of the care. We are going through that right now.”

For the head of the CDC, there are no doubts. If there was an infection, there must have “clearly” been a breach in protocol. “Infections only occur when there’s a breach in protocol,” Dr. Thomas Frieden told CBS News. “We know from many years of experience that it’s possible to care for potentials with Ebola safely without risk to healthcare workers but we also know that it’s hard, that even a single breach can result in contamination and one of the areas that we look at closely are things like how you take off the gear that might be infected or contaminated.”

The person tested positive for a fever Friday night and has been isolated ever since. Although the worker has not been identified, and hospital officials have refused to release even basic details about the patient, such as job description or gender, there are rumors that the person is a female nurse, reports the Dallas Morning News. Residents of Marquita Avenue in Dallas woke up to news of the virus on their doorstep as officials decontaminated an apartment complex where the patient lives.

This post has been updated with new information since it was originally published.

Read more of Slate’s coverage of Ebola.