Public health officials in Texas said Thursday that as many as 100 people may have had contact with the Liberian man diagnosed with Ebola. Four of these people, at least some of whom are believed to be family members of the man, have been ordered to remain at home in an attempt to prevent the spread of the disease.

Still, authorities continued to stress that only Thomas Eric Duncan, who is the first person diagnosed with Ebola in the United States, had exhibited any Ebola symptoms.

“The only person who’s had symptoms is Mr. Duncan, who’s in the hospital,” Dallas County Judge Clay Jenkins said during a news conference Thursday afternoon. “And no one who has been around Mr. Duncan in the time he has been symptomatic has shown any indication of having contracted Ebola.”

It is unclear how many people had direct contact with Duncan, who flew on commercial flights to Dallas from Liberia last month. Authorities say the number of people who require monitoring will be much lower once that is determined.

Some students have not returned to the schools attended by five students who are believed to have had contact with Duncan, the school district reported. Attendance is down at these campuses, even as nurses have begun regularly visiting classrooms and counselors have been made available for students.

Liberian authorities said Thursday they plan to prosecute Duncan for lying on an airport questionnaire, because he said he had not cared for an Ebola patient or touched anyone who had died from the disease, according to the Associated Press.

Before leaving Liberia, Duncan had his temperature taken at the airport in Monrovia by a person who had been trained by the Centers for Disease Control and Prevention. A thermometer approved by the U.S. Food and Drug Administration showed he did not have a fever.

The screening at the airport in Monrovia, the capital, has been in place for months, Deborah R. Malac, the U.S. ambassador to Liberia, said in a telephone interview Thursday.

“They have confidence that everything that was supposed to have been done was done,” Malac said.

An immigration expert and critic of the Obama administration’s immigration policies said Thursday that Duncan should not have been issued a visitor’s visa because he presented a high risk of remaining in the country illegally. She called on Washington to follow the example of several African countries, which have recently banned all visitors from Liberia and other countries affected by the outbreak of the Ebola virus.

Jessica Vaughn, a researcher affiliated with the nonprofit Center for Immigration Studies in Washington, said U.S. officials had made one “mistake” by issuing Duncan a visa last year and a second error by allowing him to enter the country.

“If you look at his circumstances, it should have been really tough for him to qualify for a visa,” Vaughn said. She noted that Duncan reportedly was jobless, living away from his home country and had a number of relatives in the U.S. — all factors that often indicate a person is unlikely to return home after their visa expires. “He clearly appears unqualified.”

Other immigration experts disagreed with this assessment and the suggestion of a travel ban, saying the current U.S. visa screening process has been made extremely rigorous in the past decade and that international health agencies oppose shutting down travel to and from the Ebola-stricken region.

A health care worker checks the temperature of a jet arriving in Monrovia, Liberia. (Michel du Cille/The Washington Post)

Doris Meissner, who headed the federal immigration agency in the 1990s and is now a senior fellow at the Migration Policy Institute in Washington, argued against banning travel from the region. She noted that the World Health Organization and other health agencies have said that closing borders and banning travel can be counterproductive because they “weaken the ability to mobilize and respond” to a health crisis.

Additional screening protocols have been added since the beginning of the Ebola outbreak, which has killed more than 3,300 people in West Africa, according to the World Health Organization.

Duncan began showing symptoms about four or five days after arriving, said Thomas Friden, director of the CDC. He visited Texas Health Presbyterian in Dallas shortly thereafter for medical treatment because he had a fever and some abdominal pain, telling a nurse that he had traveled from Liberia, but that information was not relayed to the other health care workers and he was released.

“This is a very sophisticated hospital,” said David Lakey, commissioner of the Texas Department of State Health Services, during the same conference call. “They’ve done a lot of education related to preparedness for Ebola…Unfortunately, connections weren’t made related to travel history and symptoms.”

As a result, Duncan left the hospital during the period when health officials say he was symptomatic, which is the only time Ebola becomes contagious. A little more than two days later, he returned to the same hospital in an ambulance and was placed in isolation after being recognized as a potential Ebola patient. He vomited outside an apartment complex as he was being taken into the ambulance, Reuters reported.

Duncan’s nephew told NBC News that he had to call the CDC himself to report the possible Ebola infection on Sunday. Frieden said the CDC was not aware of such a contact, but said they were looking into it.

The CDC says the list of 100 people being assessed includes “potential, possible contacts.” Many, but not all, of these people had been interviewed by Thursday, Thomas Frieden, director of the CDC, said in a conference call with reporters. The CDC has a team of 10 people, including five “disease detectives,” on the ground in Texas to help with the response and identify anyone who may have come into contact with Duncan.

Boxes of food are delivered to the apartment where Duncan was staying. (Larry W. Smith/EPA)

Four of the people on this list, at least two of whom are relatives of Duncan’s, have been ordered by state and county health officials to remain home until at least Oct. 19, with a local law enforcement officer stationed there to enforce this order. The order, which was delivered by local health officials on Wednesday night, says that they cannot have visitors without approval, has to provide blood samples and must agree to any testing.

While authorities seemed reluctant to go into detail about why these four people were quarantined, they said it had to do with making sure they remained at home and accessible for monitoring.

“They were non-compliant with the request to stay home,” Jenkins said. “I don’t want to go too far beyond that.”

Jenkins said that while the order was unusual, it was necessary in this case, adding that it was important not to have “people leaving the premises on a regular basis” against the wishes of authorities.

“We do not intend to have to do that again,” he said. “But there’s nothing more important than keeping you safe.”

People who have come into direct contact with an Ebola patient who has symptoms must be watched for three weeks, beginning on the last day they had that contact. This process, which is called contact tracing, involves observing them for symptoms such as a high fever, at which point a person will be isolated.

“These individuals do not have any symptoms,” Lakey said. “At this time, they are healthy. There’s no risk they have spread disease to any other individual.”

But he later said, “The information I was given, I couldn’t be confident that that monitoring was going to take place the way I needed it to take place.”

There are also issues of hygiene at this apartment, including properly disposing of the sheets on which Duncan slept and his belongings. The home had not been cleaned by Thursday afternoon, as there has “been a little bit of hesitancy” in finding someone willing to do it, Lakey said.

The sheets and Duncan’s belongings have been placed into a sealed plastic bag, and they will be disposed of by a contractor who agreed to clean the home and who has worked with hospitals on medical clean-ups before, Jenkins said.

Three other people who came into contact with Johnson are the Dallas Fire Rescue crew members who took Duncan to the hospital. They are going to remain home and will be checked for symptoms over the same three-week period.

If another case of Ebola does occur in the Dallas area, emergency rooms in the county are prepared to handle it, said Zachary Thompson, director of Dallas County Health and Human Services.

“The plain truth is, we can’t make the risk zero until the outbreak is controlled in West Africa,” Frieden said. “What we can do is minimize that risk, as is being done now in Dallas.”

President Obama called Dallas Mayor Mike Rawlings to discuss the response the Ebola case and pledged and reiterated his confidence in America’s doctors and national health infrastructure to handle this case safely and effectively, according to White House spokesman Eric Schultz.

Duncan flew on two commercial planes that landed at two of the busiest airports in the U.S. on Sept. 20. He boarded a United Airlines flight from Brussels to Dulles International Airport before changing planes to board another United flight to Dallas-Fort Worth. But authorities said this posed no danger to his fellow travelers or anyone who later boarded those planes, because he was not symptomatic and therefore not contagious.

As they have since Duncan was diagnosed Tuesday, public health officials assured the public that they could contain the virus.

“The bottom line here is that we remain confident that we can contain any spread of Ebola in the United States,” Frieden said. “There could be additional cases who are already exposed. If that occurs, systems are in place.”

Amy Ellis Nutt in Dallas, Tex., Pam Constable, Katie Zezima and Kevin Sieff contributed to this report.