(AP) -- A Doctors Without Borders physician who recently returned to the city after treating Ebola patients in West Africa has tested positive for the virus, according to preliminary test results, city officials said Thursday. He's the fourth confirmed case in the U.S. and the first in the nation's biggest city.

A further test by the Centers for Disease Control and Prevention will be necessary to confirm the initial test results.

A law enforcement official and a city official received notification of the preliminary test results and told The Associated Press on the condition of anonymity Thursday night but weren't authorized to discuss the case publicly before a city news conference.

Craig Spencer, a 33-year-old emergency room doctor, returned from Guinea more than a week ago and reported Thursday coming down with a 103-degree fever and diarrhea. He was rushed to Manhattan's Bellevue Hospital, a designated Ebola center, and was being treated in a specially built isolation ward.

The CDC has dispatched an Ebola response team to New York, and the city's disease detectives have been tracing the doctor's contacts to identify anyone who may be at risk.

City officials say Spencer acknowledged riding the subway and taking a cab to a Brooklyn bowling alley in the past week before he started showing symptoms.

His Harlem apartment was cordoned off, and his fiancee, who was not showing symptoms, was being watched in a quarantine ward at Bellevue. The Department of Health was on site across the street from the apartment building Thursday night, giving out information to area residents.

Health officials say the chances of the average New Yorker contracting Ebola, which is spread through direct contact with the bodily fluids of an infected person, are slim. Someone can't be infected just by being near someone who's sick with Ebola. Someone isn't contagious unless he is sick. Symptoms are similar to malaria and cholera.

The Ebola epidemic in West Africa has killed about 4,800 people. In the United States, the first person diagnosed with the disease was a Liberian man, who fell ill days after arriving in Dallas and later died, becoming the only fatality. Two nurses who treated him were infected and are hospitalized.

Mayor Bill de Blasio said proper protocols were followed every step of the way in Spencer's case and it didn't appear he had been showing symptoms for very long.

"The patient is in good shape and has gone into a great deal of detail with our personnel as to his actions the last few days so we have a lot to work with," de Blasio said earlier in the day. "We have a patient who has been very communicative and precise and who has only been back a very short time and has been quite clear about individuals he had close contact with."

According to a rough timeline provided by city officials, Spencer's symptoms developed Wednesday, prompting him to isolate himself in his apartment.

When he felt worse Thursday, he and his fiancee made a joint call to authorities to detail his symptoms and his travels. EMTs in full Ebola gear arrived and took him to Bellevue in an ambulance surrounded by police squad cars.

Doctors Without Borders, an international humanitarian organization, said per the guidelines it provides its staff members on their return from Ebola assignments, "the individual engaged in regular health monitoring and reported this development immediately." As of Oct. 14, the organization said 16 staff members have been infected and nine have died.

Spencer works at NewYork-Presbyterian/Columbia University Medical Center. He had not seen any patients or been to the hospital since his return, the hospital said in a statement, calling him a "dedicated humanitarian" who "went to an area of medical crisis to help a desperately underserved population."

Four American aid workers, including three doctors, were infected with Ebola while working in Africa and were transferred to the U.S. for treatment in recent months. All recovered.

Health care workers are vulnerable because of close contact with patients when they are their sickest and most contagious. In West Africa this year, more than 440 health workers have contracted Ebola and about half have died.

Spencer is from Michigan and attended Wayne State University School of Medicine and Columbia's University Mailman School of Public Health.

According to his Facebook page, he left for West Africa via Brussels in mid-September. A photo shows him in full protective gear. He returned to Brussels Oct. 16.

"Off to Guinea with Doctors Without Borders," he wrote. "Please support organizations that are sending support or personnel to West Africa, and help combat one of the worst public health and humanitarian disasters in recent history."

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Associated Press writers Frank Eltman, Cameron Young, Jake Person, Tom Hays and Deepti Hajela and researcher Susan James contributed to this report.