Health officials say about 80 people are being monitored for signs of Ebola infection after they had contact with a patient in Texas or the patient's family members.
Health officials haven't provided patient details for privacy reasons. However, a family friend, Gee Melish, identified the man as Thomas Eric Duncan.
"The number of people who are now part of the contact investigation has grown to more than 80," said Erikka Neroes, a spokeswoman for Dallas County Health and Human Services.
Thursday’s updated total exceeds the 18 people cited as contacts the day before.
While the patient originally told a nurse about visiting from Liberia, that information wasn’t shared with the treatment team, Dr. Mark Lester, who works for the hospital's parent company, told reporters on Wednesday.
The patient was sent home late last Thursday and returned to hospital on Sunday in an ambulance.
Authorities in the U.S. and Canada urge health-care workers to screen patients for signs of illness. They stress the importance of taking a travel history.
"Unfortunately, that did not happen in this case," said Dr. Anthony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases. "We just need to put that behind us and look ahead and make sure that in the future that doesn't happen again."
"This will certainly serve for the rest of a country as a cogent lesson learned," he added in an interview on MSNBC.
The New York Times said Duncan, who is in his mid-40s, helped to transport a pregnant woman suffering from Ebola to a hospital in Liberia, where she was turned away for lack of space. He then helped to carry her home, the newspaper said.
Health officials have emphasized that the illness is not airborne or spread through casual contact. Rather, the infection spreads through direct contact with bodily fluids.
As of Sept. 28, the Ebola virus has infected 7,178 people and killed 3,338 in Guinea, Liberia, and Sierra Leone. The World Health Organization notes that the numbers are likely widely unreported.