A doctor who was flown to the U.S. from Sierra Leone after contracting Ebola died at Nebraska Medical Centre, a hospital spokeswoman said on Monday.

Martin Salia, who tested negative for Ebola three days after exhibiting symptoms and then tested positive in a second test a week later, arrived in the U.S. on Saturday.

The spokeswoman, Jenny Nowatzke, said he died at about 4 a.m. on Monday.

Report says Mr. Salia, 44, was a Sierra Leone citizen and a permanent resident of the U.S.

Hospital officials said Mr. Salia’s condition was more critical than the other patients who had been treated there, possibly because of the lag in diagnosing the illness.

Mr. Salia did not work in the Ebola unit of the hospital in Freetown where he contracted the virus, but he treated patients for other illnesses.

It is unclear how he became infected.

Nebraska Medical Center is one of four healthcare centres in the U.S. equipped with bio-containment unit.

(dp/NAN)