DALLAS --

Tests confirmed the first known case of Ebola transmitted in the nation, raising questions about assurances by health officials here that the disease will be contained and any American hospital should be able to treat it.

Dr. Tom Frieden, head of the Centers for Disease Control and Prevention, said Sunday there had been a breach of protocol that led the worker to become infected while treating patient Thomas Eric Duncan, but officials are not sure what occurred. Duncan, who traveled from Liberia to visit family, did not get sick until he arrived in the U.S. He died Wednesday.

The worker, who has not been identified, has not been able to point to how the breach might have occurred.

President Barack Obama asked the CDC to quickly investigate the incident, the White House said.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases was asked on ABC's "Good Morning America" if federal health authorities should consider requiring that Ebola patients be sent only to highly specialized "containment" hospitals.

"That is something that should be seriously considered," Fauci said.

Dallas police barred entry to the health care worker's apartment complex Sunday. Officers also knocked on doors, made automated phone calls and passed out fliers to notify people within a four-block radius about the situation, although Dallas authorities assured residents the risk was confined to those who have had close contact with the two Ebola patients.

The worker wore a gown, gloves, mask and shield while she cared for Duncan during his second visit to Texas Health Presbyterian Hospital, said Dr. Daniel Varga of Texas Health Resources, which runs the hospital.

Duncan, who arrived in the U.S. from Liberia Sept. 20, first sought medical care for fever and abdominal pain Sept. 25. He told a nurse he had traveled from Africa, but he was sent home. He returned Sept. 28 and was placed in isolation because of suspected Ebola.

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Health care workers monitored after Ebola case