DALLAS — The first case of Ebola diagnosed in the U.S. has been confirmed in a man who recently traveled from Liberia to Dallas, sending chills through the area’s West African community whose leaders urged caution to prevent spreading the virus.

The unidentified man was critically ill and has been in isolation at Texas Health Presbyterian Hospital since Sunday, federal health officials said Tuesday. 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. 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. Frieden said it was not clear how the man became infected.

“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 said, adding that it was possible someone who has had contact with the man could develop Ebola in the coming weeks.