The 15 water-cooling towers that were found to be contaminated this week amid a new cluster of Legionnaires’ disease cases had been disinfected less than two months ago, New York City officials said on Thursday, raising questions about how successful the city can be in containing the disease.

After an outbreak of the disease killed 12 people in July and August in the South Bronx, the city required every building with cooling towers, a common source of the Legionella bacteria that cause the disease, to be cleaned within two weeks.

Despite that order, as well as new legislation mandating quarterly inspections of cooling towers, the city found this week that bacteria had regrown in at least 15 towers that had been cleaned recently in the Morris Park section of the Bronx. The testing occurred after a fresh outbreak in that area that has killed one person and sickened at least 12, and spurred an order from health officials for the towers to be disinfected again.

Building owners have complained about the costs of the mandatory cleanings. Disinfection specialists said the bacteria’s quick return was not surprising because the bacteria thrives in warmer weather, and supported the notion that the cleanings were only a short-term fix.