Health authorities in northern Italy are investigating whether bacteria—including the one that causes Legionnaires' disease—inhaled from the water supply is behind an outbreak of pneumonia that has afflicted nearly 150 people.

Experts said Monday that a single contamination but spread out over several towns and villages in Brescia was likely responsible since the cases were mostly reported in a short timeframe, not the usual person-to-person transmission that would drag out during a typical outbreak.

At least two of the cases reportedly involved victims with Legionnaire's disease—a severe form of pneumonia that is usually caused by inhaling droplets from a contaminated water source such as air conditioning systems or cooling towers. Pneumonia is typically caused by viruses, bacteria or fungi and is spread by airborne droplets. It is usually treated with antibiotics.

Brescia prosecutors on Monday opened an investigation, the ANSA news agency reported.

The Brescia public health authority issued an advisory over the weekend for residents to take basic precautions, such as changing water filters or letting water flow for some time before using it.

But the notice stressed that the water in the nine hardest hit towns and villages—many located along rivers and streams—was safe to drink. Officials were taking water samples and conducting an epidemiological investigation, the results of which were expected in a few days.

News reports say some 150 people in Brescia had come down with pneumonia, though the head of the city's health authority, Dr. Carmelo Scarcella, said the number of new cases had fallen after the peak hospital admissions were registered Sept. 2-6.

The research director for the health ministry's Superior Institute for Health, Dr. Giovanni Rezza, said he couldn't exclude that Legionnaire's bacteria was to blame.