The operator of the Texas hospital in which two nurses contracted Ebola has apologised for initially failing to diagnose the patient who infected them, and for misinforming the public about how this happened.

Dr Daniel Varga, chief clinical officer at Texas Health Resources, conceded that errors were made by Presbyterian hospital in Dallas, which sent Thomas Duncan home after he complained of a fever and abdominal pain following his arrival from his native Liberia last month.

“Unfortunately, in our initial treatment of Mr Duncan, despite our best intentions and highly skilled medical team, we made mistakes,” Varga wrote in testimony to the US Congress. “We did not correctly diagnose his symptoms as those of Ebola. We are deeply sorry.”

Duncan died on 8 October, a week after he became the first person diagnosed with Ebola in the US, and two weeks since he first sought treatment at the hospital’s emergency room. Two nurses involved in his care – Nina Pham, 26, and Amber Vinson, 29 – have since been diagnosed with the virus, raising serious questions about the hospital’s procedures.

Texas Health initially explained its failure to diagnose Duncan, who did not have health insurance, by saying that a nurse had not shared the patient’s travel records with a doctor. However, it has subsequently retracted this statement.

The Centers for Disease Control and Prevention (CDC) had advised hospitals to consider Ebola when diagnosing people suffering from a fever after travelling to countries affected by the outbreak.

Later, the hospital operator said questions over Duncan’s ability to pay had no impact on his treatment, adding that he had been cared for and prescribed antibiotics. Duncan returned to the hospital in an ambulance three days later and eventually died in an isolation unit.

Varga wrote in his testimony: “In our effort to communicate to the public quickly and transparently, we inadvertently provided some information that was inaccurate and had to be corrected.” He conceded that this had been “unsettling to a community that was already concerned and confused”.

The doctor, a senior executive vice president for the hospital operator, said he and his colleagues “don’t yet know precisely how or when” Pham was infected with Ebola as she was “using full protective measures under CDC protocols”.

The National Nurses United union criticised Texas Health for Duncan’s care after medical records suggested staff worked in the quarantine unit with their skin exposed during the three days between when the patient was admitted and when he was diagnosed. They alleged other failings in protective clothing and the isolation of Duncan.

“Were protocols breached? The nurses say there were no protocols,” the union’s co-president, Deborah Burger, said.

The hospital responded to National Nurses United’s allegations Thursday morning.

“The assertions do not reflect actual facts learned from the medical record and interactions with clinical caregivers,” it said in a statement. “Our hospital followed the Centers for Disease Control (CDC) guidelines and sought additional guidance and clarity.”

The hospital said Duncan was admitted to the emergency room and swiftly placed in isolation upon his arriving there for the second time; the union, quoting the nurses, had said he had waited for hours in a room with other people.

The hospital said staff wore the appropriate personal protective equipment as recommended by the CDC at the time, including shoe covers and face shields; the union said the caregivers who initially treated Duncan were woefully under-protected.

And the hospital said it kept up with the CDC’s evolving guidelines on personal protective equipment for staff: “When the CDC issued updates, as they did with leg covers, we followed their guidelines,” the statement said.

In an interview with the New York Times, Varga explained what he believed led to Duncan’s misdiagnosis on his initial visit to the hospital, in which he was interviewed, examined and sent home with a high fever and a course of antibiotics.

Varga said he believed a nurse didn’t ask for Duncan’s travel history until two hours after his arrival. The nurse reported that Duncan had traveled from west Africa, but she, according to Varga, did not communicate that to the treating physician who interviewed the Liberian man ten minutes later. From the interview, the doctor came away with the impression that Duncan was a local resident; Varga said he could not explain why that was.

“We didn’t correctly diagnose Mr Duncan’s initial symptoms, and we’re sorry about that,” Varga said. “It wasn’t the outcome we were hoping for.”

Varga told the paper that workers did not wear full biohazard suits during the critical three-day window before Duncan’s diagnosis was confirmed. Echoing the statement released from the hospital on Thursday, Varga said staff wore masks, gloves, gowns and shields in accordance with CDC protocols.

The 77 healthcare workers believed to have worked in the quarantine room or handled samples of Duncan’s blood are being monitored for symptoms. Varga said in his testimony the staff had been separated into three categories based on their level of risk. Those at highest risk have been told to stay off work and not use public transport. They are required to remain in the county where they live, Varga said.

In his written testimony to the subcommittee, before a hearing on Thursday, the CDC director, Dr Thomas Frieden, said the nurses’ infections “demonstrate the need to strengthen the procedures for infection-control protocols which allowed for exposure to the virus”.

It emerged on Wednesday that after suffering a fever, Vinson had asked the CDC whether she should take a flight on 13 October to Ohio, where she was to spend time with family. She had been allowed to proceed because her temperature of 99.5F (37.5C) fell below the CDC threshold of 100.4F.

CDC officials have since acknowledged that she should not have flown on a commercial plane.

Frieden, however, sought to allay public fears over the virus. He said he and his colleagues “remain confident that Ebola is not a significant public health threat to the United States” due primarily to the virus not being easily transmitted.

Vinson was transferred from Dallas to a special bio-containment unit in Atlanta on Wednesday night. Varga, who gave no update about Vinson’s condition, said: “Our thoughts and prayers are with her and her family as well.”

President Barack Obama was briefed by the relevant members of his cabinet about the country’s Ebola response on Wednesday afternoon. He also cancelled visits to Rhode Island and New York planned for Thursday in order to concentrate on the Ebola outbreak.

“I am absolutely confident that we can prevent a serious outbreak of the disease here in the United States, but it becomes more difficult to do so if this epidemic of Ebola rages out of control in west Africa,” he told reporters.