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An official at the Dallas hospital where two nurses contracted Ebola told Congress on Thursday that emergency staff were not trained this summer on how to handle the virus, despite warnings from health officials about the outbreak in West Africa.

The official, Dr. Daniel Varga, apologized to a House committee and said that authorities were still trying to determine exactly how the nurses were infected.

Both of them treated Thomas Eric Duncan, who was infected in Liberia, traveled to Dallas and showed up at Texas Health Presbyterian Hospital in late September. He was admitted only after the second visit and later died.

“It’s hard for me to put into words how we felt when our patient Thomas Eric Duncan lost his struggle with Ebola on Oct. 8,” Varga said.

“Unfortunately, in our initial treatment of Mr. Duncan, despite our best intentions and a highly skilled medical team, we made mistakes,” he said. “We did not correctly diagnose his symptoms as those of Ebola. And we are deeply sorry.”

The nurses, Nina Pham and Amber Vinson, tested positive after treating Duncan. Vinson was flown Wednesday to an Atlanta hospital unit specially equipped to handle dangerous infections such as Ebola. Pham was expected to be flown to a similar unit in at the National Institutes of Health Maryland on Thursday.

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In an exclusive interview Thursday with NBC’s TODAY, another nurse at the hospital, Briana Aguirre, said that Ebola procedures were so lax that she would refuse to go there if she developed symptoms herself.

She described a confused response at the hospital, inadequate protective gear and careless treatment of hazardous waste.

The House hearing was meant to call officials on the carpet to explain the frightening spread of Ebola in the U.S. — one imported case and now two nurses sick from that patient.

“This stakes of this battle couldn’t be any higher,” said Rep. Tim Murphy, R-Pa. “False assumptions create real mistakes, sometimes deadly mistakes.”

Varga said in prepared testimony that the hospital followed CDC and Texas state guidelines after Duncan was diagnosed and called in infectious disease specialists from the Dallas County Health and Human Services Department.

The nurses became infected despite those precautions. Varga said that the hospital has given a refresher training course to emergency department nurses and is monitoring everyone who had contact with Duncan.

The CDC director, Dr. Thomas Frieden, told the committee he feared that Ebola “could spread more widely in Africa,” and if that happened, it would present a threat to the American health care system “for a long time to come.”

As for the virus in United States, Frieden said: “There is zero doubt in my mind, unless there’s a mutation, there will not be a large-scale outbreak in the U.S.” He said that health officials had seen no sign of a mutation. Ebola spreads through direct contact with the bodily fluids of an infected person with symptoms.

Frieden and Dr. Anthony Fauci of the National Institutes of Health were asked repeatedly by some committee members why authorities shouldn’t ban travel between the United States and the Ebola-stricken countries in West Africa.

Both said, as health experts have said repeatedly, that such a ban would complicate efforts to fight the disease, not help them. And Frieden said a ban could be skirted anyway because “borders can be porous,” especially in West Africa.

Frieden said that the CDC is working to strengthen the global response to Ebola, and he added that more than 1,000 CDC staff members have helped support the effort. He is asking for money to do more, including $45 million to strength lab networks that can quickly diagnose Ebola.