DALLAS » The airline passenger who brought Ebola into the U.S. initially went to a Dallas emergency room last week but was sent home, despite telling a nurse that he had been in disease-ravaged West Africa, the hospital said Wednesday in a disclosure that showed how easily an infection could be missed.

The decision by Texas Health Presbyterian Hospital to release the patient, who had recently arrived from Liberia, could have put others at risk of exposure to Ebola before the man went back to the ER a couple of days later, when his condition worsened.

A day after the diagnosis was confirmed, a nine-member team of federal health officials was tracking anyone who had close contact with him after he fell ill on Sept. 24. The group of 12 to 18 people included three members of the ambulance crew that took the man to the hospital and a handful of schoolchildren.

They will be checked every day for 21 days, the disease’s incubation period.

"That’s how we’re going to break the chain of transmission, and that’s where our focus has to be," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, told The Associated Press Wednesday.

The CDC reminded the nation’s health care providers to ask patients with symptoms if they’ve traveled recently. The American College of Emergency Physicians planned to alert its members as well.

The patient explained to a nurse last Thursday that he was visiting the U.S. from Africa, but that information was not widely shared, said Dr. Mark Lester, who works for the hospital’s parent company.

"Regretfully, that information was not fully communicated" throughout the medical team, Lester said. Instead, the man was diagnosed with a low-risk infection and sent home.

He was prescribed antibiotics, according to his sister, Mai Wureh, who identified her brother, Thomas Eric Duncan, as the infected man in an interview with The Associated Press.

Duncan has been kept in isolation at the hospital since Sunday. He was listed in serious but stable condition.

Hospital epidemiologist Dr. Edward Goodman said the patient had a fever and abdominal pain during his first ER visit, not the riskier symptoms of vomiting and diarrhea.

But the diagnosis, and the hospital’s slip-up, highlighted the wider threat of Ebola, even in places far from West Africa.

"The scrutiny just needs to be higher now," said Dr. Rade Vukmir, a spokesman for the American College of Emergency Physicians.

Ebola is believed to have sickened more than 7,100 people in West Africa and killed more than 3,300, according to the World Health Organization. Liberia is one of the three hardest-hit countries in the epidemic, along with Sierra Leone and Guinea.

In Texas, neither the ambulance crew nor the children showed any symptoms. They were restricted to home while their conditions are observed. It was not exactly clear how Duncan knew the children, but his sister said he had been visiting with family, including two nephews.

At an address listed for the family in Kannapolis, North Carolina, a man who identified himself as Duncan’s brother told reporters the family was "just stressed out."

Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus. The disease is not contagious until symptoms begin. It spreads only by close contact with a patient’s bodily fluids.

The man left Liberia on Sept. 19, flying from Brussels to Dulles Airport near Washington. He then boarded a flight for Dallas-Fort Worth, according to airlines, and arrived the next day to see relatives.

He started feeling ill four or five days later, Frieden said.

The CDC sent a team to Monrovia’s airport on Wednesday to make sure health officials there are screening passengers properly. All people traveling from the outbreak zone are supposed to be checked for fever and asked about their travel history before being allowed to leave. Plastic buckets filled with chlorinated water for hand-washing are present throughout the airport.

"There were no signs of any disease when the gentleman boarded the flight," said Dr. Tom Kenyon, director of the CDC’s Center for Global Health. "This was not a failure of the screening process at the airport."

Since the man was showing no symptoms on the plane, the CDC stressed that there is no risk to fellow passengers.

Tests designed to detect the virus in the bloodstream are not typically performed on people who don’t yet have symptoms, because tests done too early may miss the diagnosis, Frieden said.

"Even in the initial phases of illness, when they’ve got a fever, the most sensitive tests in the world don’t detect it because there’s so little virus that they have," he said. "It’s only as they become sicker that they become more infectious."

Hospitals have been on the lookout: The CDC has received 94 inquiries from states about illnesses that initially were suspected to be Ebola, but after taking travel histories and doing some other work, most were ruled out. Of the 13 people who actually underwent testing, only one — the Dallas patient — tested positive.

The Dallas apartment complex where Duncan was believed to be staying was cordoned off Wednesday, and the management was turning away visitors. TV cameras lined the fence of the parking lot, and at least one helicopter hovered overhead.

Four American aid workers who became infected in West Africa have been flown back to the U.S. for treatment after they became sick. Three have recovered.