Centers for Disease Control and Prevention Director Tom Frieden admitted to a House panel on Thursday that officials still don’t know exactly how two nurses contracted Ebola from Thomas Eric Duncan, the first man to be diagnosed with the virus in the U.S.

Nina Pham and Amber Vinson, nurses at Texas Health Presbyterian Hospital Dallas, became ill after treating Duncan, who died earlier this month.

“While we do not yet know exactly how these transmissions occurred, they demonstrate the need to strengthen the procedures for infection-control protocols which allowed for exposure to the virus," Frieden said while testifying at a hearing of the House Subcommittee on Oversight and Investigations.

Dr. Daniel Varga, the chief clinical officer of Texas Health Resources, which oversees the hospital, joined the hearing via teleconference and admitted to several mistakes, ranging from Duncan's initial discharge from the emergency room and confusing missives about the timeline of events to the eventual transmission of the virus to the nurses.

Varga called Pham "an extremely skilled nurse" who used CDC-recommended protective measures when caring for Duncan.

"But it’s clear there was an exposure somewhere, sometime," said Varga. "We are poring over records and observations, and doing all we can to find the answers."

"We are working very hard to investigate the situation, but are not waiting for the completion of this investigation and have already helped the hospital implement new measures for safety," Frieden said.

The CDC has already recommended several policy changes for Texas Health Presbyterian Hospital, including decreasing the number of health workers who care for Ebola patients. The agency also suggested limiting staff interactions with the patients to only "essential procedures" and providing better training for how to use protective equipment.

Currently, officials are monitoring 76 people who had possible contact with Duncan when he was hospitalized, and the Texas hospital has made rooms available for any staffers who want to self-isolate, out of an abundance of caution.

"We are doing this for our employees’ peace of mind and comfort," a hospital spokesperson said. "This is not a medical recommendation."

Meanwhile, Pham and Vinson will be cared for by other hospitals that specialize in caring for patients with diseases like Ebola.

Vinson was transferred to Emory University Hospital in Atlanta on Wednesday, which has already nursed Ebola patients Dr. Kent Brantly, Nancy Writebol and an unnamed third patient back to health.