NEW YORK, N.Y. - A historic hotel's rooftop air-conditioning unit is the source of a Legionnaires' disease outbreak that has killed 12 people and sickened more than 100 in the Bronx, the city's health commissioner said Thursday.

After extensive sampling and testing of Legionnaires'-causing bacteria, laboratories have matched the strain found in the hotel's rooftop unit, also called a cooling tower, with the strain found in 25 patients, some of whom died, Health Commissioner Mary Bassett said.

"The outbreak is over," Bassett said, noting the maximum incubation period has passed since the last new report of someone coming down with symptoms.

Health officials investigating the outbreak learned nine days into their probe, on July 29, that a guest who had stayed at the Opera House Hotel had become ill, said Dr. Jay K. Varma, a deputy health commissioner for disease control.

In all, there have been 128 cases, and almost all patients hospitalized with the disease have been discharged, officials said.

The outbreak has become the city's most significant public health crisis since last fall's Ebola scare. For more than a month, cases of Legionnaires' disease, a form of pneumonia especially dangerous for the elderly and for people with underlying health issues, have been reported throughout a section of the South Bronx, the city's poorest neighbourhood.

Cooling towers in 18 buildings tested positive for the bacteria, and the outbreak prompted city and state officials to require the testing and inspection of building cooling towers across the state. The towers emanate warm mist that can spread bacteria.

Mist can travel up to a mile, under some conditions, and it's unclear how close some patients had come to the hotel, officials said. Two had stayed there; no employees were sickened.

The Opera House Hotel's building is more than a century old and once housed performances by Harry Houdini, the Marx Brothers and George Burns. It's in the heart of the South Bronx.

After speculation last week on the source of the outbreak, the hotel said that its 2-year-old cooling tower has been cleaned routinely and that managers acted quickly to get it cleaned again after learning of the Legionnaires' problem.

Hotel owners initially complained that city health officials hadn't been forthcoming. But on Thursday, the hotel, which was open, said that city and state health officials "have kept us fully informed" and pointed out that a test Wednesday was negative for the Legionnaires'-causing bacteria.

The hotel now plans to test its cooling system every 30 days when the system is in use, though new city and state requirements call for testing every 90 days.

"Given the recent events, we have decided to be especially cautious going forward," the hotel said.

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Reach Jennifer Peltz on Twitter @jennpeltz.

News from © The Associated Press, 2015