Investigators have not determined how Pham specifically contracted the disease from Duncan, who died on his 10th day of intensive care at the hospital.
"If this one individual was infected and we don't know how within the isolation unit, then it is possible that other individuals could have been infected as well," Frieden said during a press conference. "We consider them to be at risk, and we're doing an in-depth review and investigation."
[Related: About 70 hospital staffers cared for Ebola patient Thomas Eric Duncan]
A day earlier, Frieden characterized the transmission from Duncan to Pham as a possible breach in safety protocols. On Monday he apologized for those remarks.
"Some interpreted that as finding fault with the hospital or the health care worker, and I'm sorry if that was the impression given. That was certainly not my intention," Frieden said. "What we need to do, is all take responsibility for improving the safety of those on the front lines. I feel awful that a health care worker became infected in the care of an Ebola patient. She was there trying to help the first patient survive.