The number of people in Texas who are being screened for potential exposure to Ebola expanded to approximately 100, and four members of a family close to the U.S. patient were ordered to remain in their Dallas home. (Photo: AP)

The number of people in Texas who are being screened for potential exposure to Ebola expanded Thursday to roughly 100, as health officials cast a wide net to try to prevent the one confirmed case of the disease from sparking an outbreak.

Four members of a family close to Thomas Eric Duncan, the Liberian man diagnosed with the virus, were ordered to remain in their Dallas home and not receive any visitors until at least Oct. 19, to pass the 21-day maximum incubation period for the often-deadly disease.

The 100 people being screened represent a “very wide net,” including some who possibly had brief encounters with Mr. Duncan, Texas health officials said. They added that the number is likely to drop as they narrow the list to those actually at potential risk of infection.

Thursday, an American freelance journalist in Liberia tested positive for the disease, his father and his employer, NBC News, said. The 33-year-old man is tentatively scheduled to be transported back to the U.S. on Sunday.

In Mr. Duncan’s case, Tom Frieden, director of the Centers for Disease Control and Prevention, said officials so far have identified only “a handful” of individuals who may have had close contact with him.

The public health search comes as authorities in Liberia grapple with how Mr. Duncan managed to leave their country and bring Ebola to the U.S. despite government efforts to stop transmission of the virus, a journey that took him from a neighborhood of tin-roof houses in a West African capital to an isolation ward at a Dallas hospital.

Before traveling to Texas via Belgium, Mr. Duncan escorted a woman to a treatment ward in Liberia’s capital, Monrovia, where she was turned away and died of the virus within hours, said Irene Seyou, Mr. Duncan’s next-door neighbor.

In a community near where U.S. victim Thomas Eric Duncan lived in Monrovia, many have died and children are worried they will be taken away. Glenna Gordon for The Wall Street Journal

On Sept. 16, several health workers arrived in Mr. Duncan’s neighborhood in Monrovia to investigate a report that a pregnant 18-year-old woman, recently sent home from a nearby clinic, had shown Ebola symptoms that included vomiting, diarrhea and bleeding, said Prince Toe and other members of the Ebola Response Team in the capital’s 72nd community.

But when the team arrived in the neighborhood, residents insisted the pregnant teenager had been in a car accident, said Mr. Toe, the unit’s supervisor. When the neighbors grew rowdy at being pressed for information, the team turned back, he said.

At Liberia’s airport, the temperatures of arriving and departing passengers are checked three times by security guards—at the entrance, before the check-in desk and at the metal detectors—to screen out those who display Ebola’s hallmark early symptom, a fever.

Passengers are asked to fill out questionnaires about whether they had been in contact with any Ebola victims. Mr. Duncan lied on those forms—and would be prosecuted for doing so if he returns to Liberia—the Associated Press reported Liberia’s government as saying Thursday.

Mr. Duncan is in an isolation unit at Texas Health Presbyterian Hospital in Dallas, which initially sent him home with antibiotics after he complained of illness, only to accept him on Sunday after he returned in an ambulance. Hospital officials have since conceded that they erred by not taking him in initially after he mentioned his symptoms and country of origin.

Hospital officials said Thursday that Mr. Duncan’s condition continued to be serious. Dr. Frieden of the CDC said Mr. Duncan’s physicians were discussing the possible use of experimental treatments with his family.

Edward Goodman, Texas Health Presbyterian Hospital’s epidemiologist, said the team of doctors treating Mr. Duncan has received guidelines from the CDC but that there is no specific treatment for Ebola other than supportive measures, such as keeping the patient well hydrated to avoid organ damage and supplying oxygen.

Most of the 100 people Texas is tracking for potential Ebola exposure haven’t been ordered to stay home. Officials said they ordered four of Mr. Duncan’s family members to remain in their home because the family disobeyed their request to stay there. They said the family, which was examined Thursday, hadn’t developed any symptoms. A law enforcement official is stationed outside their apartment to make sure they don’t leave.

Ebola is a highly contagious virus, but only if you come into contact with certain bodily fluids of those infected. What do scientists know about how it’s transmitted? WSJ’s Jason Bellini has #TheShortAnswer.

Judge Clay Jenkins, the highest elected official in Dallas County, said there were no plans to issue similar orders for other people. Local and state health officials said they had delivered groceries to the family and were arranging for a contractor to clean the apartment. Mr. Jenkins said it appeared that sealed bags filled with Mr. Duncan’s belongings, including his clothes and sheets, were still inside, and that the family had pushed mattresses against the wall.

Dallas Mayor Mike Rawlings sought to assure the public that the risk of contagion was minimal. “We’re getting the word out and people are starting to understand what has happened,” he said.

Still, at schools attended by five children who came into contact with Mr. Duncan, attendance was down to 86% from the 95% level that is normal, said Mike Miles, superintendent of the Dallas Independent School District, who added that custodians were doing extra cleaning.

While officials sought to control the panic over Ebola in Texas, some people who had come into contact with Mr. Duncan wondered why he hadn’t received treatment sooner.

Joe Joe Jallah said he met Mr. Duncan last week when visiting Mr. Jallah’s former wife, Louise Troh, the same woman Mr. Duncan had come to see in the U.S.

Ms. Troh declined to speak about the situation when reached by phone.

Mr. Jallah, who has a daughter with Ms. Troh, said he listened as Mr. Duncan described how dire things had become in Liberia, and how rigorous the health screenings were during his trip to the U.S.

Several days later, on Saturday, Mr. Jallah said he heard that Mr. Duncan had fallen ill at Ms. Troh’s apartment. Concerned, Mr. Jallah went back.

“He was lying down on the floor with a comforter. He said he was sick and that he had no appetite,” Mr. Jallah said.

“I said, ‘Did you go to the hospital?’ He said, ‘Yes, but they did nothing for me,’” Mr. Jallah recalled. “I said, ‘You should eat so you can gain strength.’”

The next day, Mr. Jallah said he returned after his daughter, Youngor Jallah, a nurse’s aide who visits her mother frequently, called, sounding frantic and saying that Mr. Duncan was still sick.

Ms. Jallah said Mr. Duncan had been up all night with diarrhea. His eyes were red, he seemed exhausted and had no appetite for the breakfast she made. He tried to drink some tea. Ms. Jallah took his temperature and it was 104, she said.

Ms. Jallah decided to call an ambulance. When emergency workers came, she informed them that Mr. Duncan was sick and had traveled to Dallas from a virus stricken-region in Africa. The workers put masks over their faces.

Ms. Jallah said she has since been told she and her family must stay in their home for 17 more days.

“I am concerned for myself. When I took his blood pressure, I never had no protection. I worry about my kids. My kids were over there with my mom,” she said.

“I am worried about him too,” she added.