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 those people, at least two of whom are family members, 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, the first person to be diagnosed with Ebola in the United States, had exhibited any symptoms of the disease.

“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, where county officials provided an update on their response. Jenkins is the top elected official in Dallas County.

“And no one who has been around Mr. Duncan in the time he has been symptomatic has shown any indication of having contracted Ebola,” Jenkins said.

The Centers for Disease Control and Prevention said 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 Ebola outbreak in West Africa has reached the United States, as officials confirm one case in Dallas. Here's how U.S. health officials plan to stop the virus. (Gillian Brockell/The Washington Post)

It is unclear how many people had direct contact with Duncan. Authorities say the number of people who require monitoring will be much lower once that has been 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. Those five students are staying at home, and attendance is down at the campuses, even as nurses have begun regularly visiting classrooms and counselors have been made available.

A law enforcement officer was stationed at the apartment complex Duncan was visiting to make sure the quarantined individuals do not go out.

The quarantine order, which was delivered by local health officials Wednesday night, says those inside cannot have visitors without approval, have to provide blood samples and must agree to any testing.

While authorities were reluctant to go into detail about why these four people without any symptoms were quarantined, they said it had to do with making sure they remained at home and accessible for monitoring.

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

There are also issues of hygiene at the apartment, including properly disposing of Duncan’s belongings and the sheets on which he slept. The home had not been cleaned by Thursday afternoon because there has “been a little bit of hesitancy” in finding someone willing to do it, said David Lakey, commissioner of the Texas Department of State Health Services.

1 of 66 Full Screen Autoplay Close Oct. 9, 2014 Oct. 7, 2014 Oct. 6, 2014 Oct. 4 to Oct. 5, 2014 Oct. 3, 2014 Agence France-Presse via Getty Images" data-title=" Oct. 3, 2014" data-max-width="4000" data-image="https://www.washingtonpost.com/rw/2010-2019/WashingtonPost/2014/10/04/Health-Environment-Science/Images/Par7992489.jpg?env=A" data-ratio="1.477"> Oct. 2, 2014 Sept. 27, 2014 Sept. 24, 2014 Sept. 20, 2014 Skip Ad × Struggling to contain Ebola epidemic in Africa View Photos As the overall death toll has risen to more than 3,000, the first U.S. case of Ebola is diagnosed in Texas. Caption The death toll has risen to more than 4,000 in West Africa, with the first U.S. death from disease occurring in Texas. Oct. 11, 2014 Staff from the North East Ambulance Service and the Royal Victoria Infirmary in Newcastle take part in a national exercise to test Britain’s readiness for an Ebola outbreak. Andrew Fox/Britain’s Department of Health via Reuters Buy Photo Wait 1 second to continue.

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

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

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. If a contact exhibits a high fever, that person will be isolated.

Liberian authorities said Thursday they plan to prosecute Duncan for lying on an airport questionnaire when he said he had not cared for an Ebola patient or touched anyone who had died from the disease, according to the Associated Press. Neighbors in the Liberian capital said they thought he became infected after he helped transport a sick, pregnant neighbor, the AP said.

Before Duncan left Liberia, his temperature was taken at the airport in Monrovia by a person trained by the CDC. 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, Liberia’s 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.

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 in Texas, Frieden said. He visited Texas Health Presbyterian Hospital Dallas shortly thereafter for medical treatment because he had a fever and some abdominal pain.

He told a nurse that he had traveled from Liberia, but that information was not relayed to other health-care workers, and he was released from the hospital.

“This is a very sophisticated hospital,” Lakey said during the 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 is 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.

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.”

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

As they have done since Duncan’s illness 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.”

mark.berman@washpost.com

Amy Ellis Nutt in Dallas and Kevin Sieff contributed to this report.