An October 2013 photo of Nancy Writebol with children in Liberia. (Courtesy of Jeremy Writebol/via AP)

The second American known to be infected with the Ebola virus arrived back in the United States on Tuesday.

Nancy Writebol, a missionary from Charlotte, N.C., was flown in the same “air ambulance” that returned the first Ebola-stricken American to the country over the weekend. Kent Brantly, an American doctor who contracted the virus in West Africa, was flown to the U.S. on Saturday and taken to Emory University Hospital in Atlanta.

She arrived at the same hospital on Tuesday after first flying to Maine. The plane carrying Writebol from Liberia arrived at Bangor International Airport at 8:09 a.m. and, after clearing customs, took off for Atlanta about half an hour later, reported the TV station WCSH in Portland, Me.

Writebol’s condition appears to be improving, according to Mission, the Christian relief group that employs Writebol and her husband.

“Nancy is still very, very weak, but shows continued, but slow improvement,” Bruce Johnson, president of SIM USA, said in a news conference. “She is showing signs of progress and moving in the right direction.”

Brantly and Writebol had been treating Liberian victims of the largest Ebola outbreak in history. So far, more than 1,600 people have been sickened by Ebola in the outbreak and at least 887 people have died, according to the World Health Organization.

The two Americans were given doses of an unproven serum meant to fight the disease. A spokesman for SIM told The Washington Post that her condition seemed to stabilize in the days after she received the serum.

Emory University Hospital has a special isolation unit that was created with the CDC to provide treatment for patients who encounter certain infectious diseases. This facility is separated from the other patient areas and is incredibly isolated, according to Emory.

Writebol’s arrival comes a day after New York’s Mount Sinai Hospital tested a patient who had symptoms consistent with Ebola and who had recently visited a West African country. The man was placed in isolation and screened, with city health officials saying on Monday evening that the man was “unlikely to have Ebola.”

The CDC issued a travel warning last week telling people to avoid non-essential trips to Guinea, Liberia and Sierra Leone.

This post has been updated.