DALLAS — The first case of Ebola diagnosed in the U.S. was confirmed Tuesday in a patient who recently traveled from Liberia to Dallas — a sign of the far-reaching impact of the out-of-control epidemic in West Africa.

The unidentified man was critically ill and has been in isolation at Texas Health Presbyterian Hospital since Sunday, federal health officials said. They would not reveal his nationality or age.

Authorities have begun tracking down family, friends and anyone else who may have come in close contact with him and could be at risk for becoming ill. But officials said there are no other suspected cases in Texas.

At the Centers for Disease Control and Prevention, Director Tom Frieden said the man left Liberia on Sept. 19, arrived the next day to visit relatives and started feeling ill four or five days later. He said it was not clear how the patient became infected.

There was no risk to any fellow airline passengers because the man had no symptoms when he was traveling, Frieden said.

Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus. The disease is not contagious until symptoms begin, and it takes close contact with bodily fluids to spread.

“I have no doubt that we’ll stop this in its tracks in the U.S. But I also have no doubt, that as long as the outbreak continues in Africa, we need to be on our guard,” Frieden told reporters.

“It is certainly possible that someone who had contact with this individual, a family member or other individual, could develop Ebola in the coming weeks,” he added. “But there is no doubt in my mind that we will stop it here.”

He said he didn’t believe anyone on the same flights as the patient was at risk.

“Ebola doesn’t spread before someone gets sick and he didn’t get sick until four days after he got off the airplane,” Frieden said.

Frieden briefed President Barack Obama by phone about the diagnosis, the White House said.

Word of the infection alarmed the local Liberian community.

“People have been calling, trying to find out if anybody knows the family,” said Stanley Gaye, president of the Liberian Community Association of Dallas-Fort Worth. “We’ve been telling people to try to stay away from social gatherings.”

Four American aid workers who became infected in West Africa have been flown back to the U.S. for treatment after they became sick. They were cared for in special isolation facilities at hospitals in Atlanta and Nebraska. Three have recovered.

Also, a U.S. doctor exposed to the virus in Sierra Leone is under observation in a similar facility at the National Institutes of Health.

The U.S. has only four such isolation units. Asked whether the Texas patient would be moved to one of those specialty facilities, Frieden said there was no need and virtually any hospital can provide the proper care and infection control.