U.S. health experts in Dallas were taking stock Wednesday of how many people may have been exposed to Ebola, just a day after the first case of the deadly virus was diagnosed in the United States, the nation's top public health official said.

The review comes even as health officials in Texas said healthcare workers tested negative for the virus and there were no other suspected cases in the state.

Health officials confirmed the first case of the virus in the United States on Tuesday, when a man who flew from Liberia to Texas tested positive for Ebola, which has killed more than 3,000 people in West Africa.

"We have a seven-person team in Dallas today helping to review that with the family and make sure we identify everyone that could have had contact with him," Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention (CDC), told NBC in an interview.

Frieden said experts were monitoring "a handful" of people who were potentially exposed through physical contact with the patient. Officials were looking at family members the patient visited, as well as healthcare providers who helped treat him.

Dallas city officials on Wednesday said the three ambulance crew members who transported the man tested negative for the virus, although they were quarantined and would be closely monitored for the next 21 days, the amount of time it can take for symptoms to appear.

Texas state health officials, in a post on Twitter, said there were no other suspected cases of Ebola at this time.

"The team on the ground will review that very intensively to see whether there's any other groups who, out of an abundance of caution, we would want to monitor carefully," Frieden told NBC's "Today" show from CDC headquarters in Atlanta.

Ebola spreads through contact with bodily fluids like blood or saliva, which health experts say limits its potential to infect others, unlike airborne diseases.

Still, the long window of time before patients exhibit signs of infection, such as fever, vomiting and diarrhea, means an infected person can travel without detection.

The virus can be fatal. While past outbreaks killed as many as 90 percent of victims, the current world outbreak has a fatality rate of about 50 percent.

The patient in the U.S., who was not identified for privacy reasons, arrived in Texas on Sept. 20, and sought treatment six days later at Texas Health Presbyterian Hospital in Dallas, according to the CDC.

He was initially evaluated and sent home with antibiotics, but returned two days later. He was then admitted and isolated, a delay that has been questioned by other health experts.

On Wednesday, Frieden and other officials reiterated their call for heathcare workers to be vigilant in screening patients in the United States for possible signs of the virus.

"If you have someone who's been in West Africa in the past 21 days and they've got a fever or other symptoms that might be consistent with Ebola, immediately isolate them, get them tested," Frieden told NBC.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Disease, said on CNN that emergency room personnel should obtain information on patients' travel history when they show possible symptoms, calling "an enormous red flag."

U.S. health authorities have said every step was being taken to ensure the Ebola virus does not spread widely in the United States and have expressed confidence that it can be contained.

Health officials and lawmakers have been prepared for the possibility that a patient would arrive on U.S. shores undetected. Frieden briefed President Barack Obama on the Ebola issue Tuesday and they discussed isolation protocols.

"People can be confident here in this country that we have the medical infrastructure in place to prevent the broad spread of Ebola," White House spokesman Josh Earnest told CNN on Wednesday. "The CDC and the local health officials in north Texas are taking the responsible steps to ensure the safety of the broader public."

Some other health experts have said, given the information from the CDC so far, a widespread outbreak in the United States appears unlikely from this single case.

They note that doctors, nurses and other healthcare providers routinely use gloves, masks and gowns when examining patients.

Tom Solomon, an emerging infections expert at the University of Liverpool in Britain, said while it was "worrying" that Ebola had spread to America, "the chances of it becoming established in America or other Western Countries is very small."

"While it's not impossible to transmit it, it's not going to breakout and transmit (to) a whole neighborhood or a whole city," Dr. Ezekiel Emanuel, a former White House health adviser, told MSNBC.