United Airlines is scrambling to alert the hundreds of passengers who shared two planes with the first Ebola patient on U.S. soil.

Thomas Eric Duncan, a Liberian national, arrived in the U.S. at Washington Dulles International Airport on United Flight 951 on September 20, the airline revealed today.

He then boarded United Flight 822 to Dallas/Fort Worth International Airport.

U.S. officials had previously refused to provide details of his itinerary, claiming none of his fellow passengers were at risk because he was not showing symptoms at that time.

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Thomas Eric Duncan flew from Liberia to Brussels and then into Dallas/Fort Worth International Airport - with a connection in between

Unfriendly skies: Thomas Eric Duncan was able to board a plane to the U.S. and land in Dallas after having come in close contact with at least one person who died of Ebola

United Airlines released the details of Duncan's journey on Wednesday night. He left September 19 from Monrovia, Liberia, and flew to Brussels. From Brussels, he caught a United Airlines flight to the U.S.

The revelation that Duncan entered the United States on one side of the nation then boarded a domestic flight to Texas shows just how quickly the disease can spread if infected patients are not stopped at the border.

The New York Times reports that Duncan helped care for a woman who later died of the disease just days before his journey. Had a customs official asked him about his contact with Ebola patients, he could have been flagged as being at high risk for infection.

Still, public health officials say the chances of an outbreak in the United States are low. The United States has sanitation and medical care that far exceeds that found in the affected West African nations, which are among the poorest in the world.

Currently, patients are checked for fever - the first sign of a possible infection - before they are allowed to board flights leaving West African nations.

However, once travelers gets past examiners in Liberia, Sierra Leon or Guinea, there are few barriers to them moving across the world.

U.S. Customs and Border Protection says the agency's examiners at airports and other ports of entry are trained to spot and identify people potentially infected with Ebola before they enter the country.

It's a long way to Texas: Thomas Duncan flew from Liberia to Belgium to Washington, D.C., before finally arriving in Dallas on September 20

However, a spokeswoman could provide no information about how the agency was changing its rules to prevent more Ebola infected fliers from coming into the U.S.

Airlines for America, the industry trade group, on Wednesday U.S. airlines are in regular contact with the Centers for Disease Control and other government agencies about helping to prevent the spread of Ebola.

'A4A members that fly to affected countries remain in steady contact with government agencies and health officials, and have procedures in place to monitor and quickly respond to potential health concerns,' spokeswoman Victoria Day said.

Stocks in major U.S. air carriers fell as much as nearly 4 percent on Wednesday over fears that the spread of the worst known Ebola outbreak beyond West Africa would make more customers fearful of traveling.

'People are nervous about (the first case of Ebola detected in the United States) and what it means,' said Michael Derchin, an analyst at CRT Capital Group LLC.

Still, Derchin said that the market had overreacted.

'I would be surprised if there's any impact on travel,' he said.

JetBlue Airways and American Airlines said they were closely following guidelines from the U.S. Centers for Disease Control and Prevention (CDC).

Airlines for America, the industry trade group, is also coordinating with the CDC on any government action related to Ebola concerns, JetBlue spokesman Morgan Johnston said.

'We follow the guidelines (put) in place by the CDC specifically for airlines, and we work with our crews to protect the health of our customers and employees,' American Airlines spokesman Josh Freed said.

The CDC is working with U.S. Customs and Border Protection (CBP) to identify potential victims and warn other travelers of the health threat.

'CBP personnel receive training in illness recognition,' said agency spokeswoman Jennifer Evanitsky. If they identify someone believed to be infected, they will seek medical evaluation from CDC and local health officials.

Customs officials will wear protective equipment such as gloves and surgical masks to ensure their safety when interacting with ill travelers, she added.

Customs and Border personnel also will hand out flyers in airports encouraging people to watch their health for 21 days and listing steps to follow should they become sick, according to CDC spokesman Tom Skinner.

Airlines will also remind their customers to follow CDC guidelines regarding travel when ill, Johnston said.