Precisely how and where the 86 people with Legionnaires’ in the current South Bronx outbreak contracted it remained under investigation by health authorities. But five water cooling towers — a component of the heating, ventilation and air-conditioning systems in many modern buildings — have tested positive for legionella in the affected area and are thought to be the source of the outbreak. That finding has highlighted longstanding concerns about the upkeep and oversight of the cooling towers, which provide the damp, warm environment that the bacteria need to thrive and must be cleaned regularly to prevent bacteria from taking root.

Dr. Jay Varma, deputy commissioner for disease control for the city health department, said in an interview that the city had not proposed regulation sooner in part because identifying the cause of the disease can be difficult in many cases. He said it was also not known how often cooling towers were the source. “The challenge is we don’t know where most infections normally come from,” he said.

In the South Bronx, Dr. Varma said, officials are confident that they have identified and addressed the likeliest cause of the outbreak but have yet to determine whether it was one or more of the towers. Citing privacy laws, he said the city would not release the names of the dead.

At the news conference on Tuesday, Mr. de Blasio said the city planned to tighten regulation of the towers, but in recent days, community leaders, neighborhood residents and industry experts have faulted the city for failing to have a more rigorous inspection regime in place.