Mayor Bill de Blasio said on Thursday that city health officials are investigating whether there is a connection between new cases of Legionnaires' disease in the Bronx and an outbreak earlier this summer that killed 12 people. 

The Health Department said four cases of Legionnaires' disease had been identified at the Melrose Houses in the South Bronx; one person has been hospitalized. 

Health Commissioner Mary T. Bassett had announced Wednesday that a preliminary test found the presence of Legionella pneumophila bacteria in the water system of one of several buildings, which don't have cooling towers. Two other buildings tested negative and tests of five others were still being processed. 

The Melrose Houses, an eight-building public housing complex with more than 1,000 residential units, covers more than 12 acres. 

The Health Department said it has moved to shut off hot water in the building at 681 Courtlandt Ave., where a test revealed the presence of the bacteria. Health officials said the department will work to install water filters in every apartment unit. 

Health officials, who are coordinating with the U.S. Centers for Disease Control and Prevention, are also working on "long-term plans for disinfection." 

The city's deadliest-ever Legionnaires' disease outbreak, which killed 12 people and sickened 116 others in the South Bronx earlier this summer, was declared over on Aug. 20. The source of the outbreak had been traced to the cooling tower of the Opera House Hotel on 149th Street. 

No one had reported the onset of Legionnaires' symptoms since Aug. 3. Symptoms, which include cough, shortness of breath, high fever, muscle aches and headaches, usually begin two to ten days after exposure to the bacteria. 

The disease is contracted through inhaling mist or vapor from contaminated water systems and cooling towers, but is not contagious. It is treated with antibiotics.