A New York City doctor who recently treated Ebola patients in West Africa has tested positive for the disease, city and state officials announced Thursday.

Craig Spencer, 33, reported symptoms for the first time Thursday morning, one week after returning from work at an Ebola treatment center in Guinea. 

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Four people have been isolated because of potential exposure to Spencer. Three of them are being quarantined, Gov. Andrew Cuomo said during a press conference Thursday night.

Spencer arrived at Bellevue Hospital “immediately” after noticing symptoms, which included a 103-degree fever and gastrointestinal problems. He had been self-monitoring for symptoms since returning from Guinea, New York City Health Commissioner Dr. Mary Bassett said Thursday night.

“He was being mindful about contact with people,” she said, adding that he spent "most of his time" self-isolating in his apartment.

Spencer is the third traveler who has unknowingly carried Ebola outside of Africa and the second person who has brought it to the U.S. He had treated Ebola patients as part of his work with Doctors Without Borders.

The diagnosis raises extra concerns because Spencer traveled across New York City on the subway and went to a bowling alley on Wednesday night, just hours before he noticed symptoms. One of the people who is being monitored is an Uber driver, though he did not have direct contact with Spencer.

The bowling alley, which is called The Gutter and located in Brooklyn, has been temporarily shut down, officials said. They added that, while Spencer was feeling fatigued at the time, he was not symptomatic and, therefore, could not have spread the disease.

During the press conference, New York officials stressed that people carrying Ebola cannot spread the disease unless they are showing symptoms. Even then, the disease only spreads through contact with bodily fluids.

Hospitals in the U.S. have already treated six patients for Ebola; five have recovered or are on their way to recovery.

Cuomo said New York officials began preparing for a possible Ebola case immediately after the country’s first diagnosis in Dallas on Sept. 30. That patient died about a week later, after a series of missteps by the hospital.

“We are as ready as one could be for this circumstance. What happened in Dallas was actually the exact opposite,” Cuomo said. “We had the advantage of learning from the Dallas experience.”

After that patient's death, the Obama administration laid out stricter rules for treating patients, vowing to be hands-on during the treatment of anyone diagnosed in the U.S. 

Emergency response teams from Centers for Disease Control and Prevention have already been sent to New York, officials said. President Obama has also been briefed on the latest case, according to a White House official.

The Obama administration has also taken major steps to combat the threat of Ebola domestically in the last week. Officials announced Wednesday that all travelers into the U.S., including aid workers and journalists, would be monitored for three-weeks after returning from Ebola hot zones. Obama also tapped Vice President Biden's former chief of staff Ron Klain, as a coordinator to direct the government's response to the disease.

The newest Ebola case is likely to reignite a firestorm of criticism Friday morning during hearing of the House Oversight and Government Reform Committee. The hearing, which has been planned for two weeks, will evaluate the country's domestic response to Ebola.

More than 9,200 cases of Ebola have been confirmed in eight countries worldwide, though nearly all have been reported in three West African countries: Sierra Leone, Liberia and Guinea.

— Justin Sink contributed to this report, which was updated at 10:54 p.m.