“Poisoned Water,” a one-hour documentary that looks at the 2015 outbreak of Legionnaires’ disease in Flint, MI, debuts at 9 EDT tonight on the PBS science series “Nova.”

In April 2014, city and state officials switched Flint’s water supply from Detroit’s water system to the Flint River, and in so doing, decided against treating the river water with phosphates, an anti-corrosive agent. That decision helped produce a situation in which lead leached into the water.

In March of this year, Virginia Tech Professor Marc Edwards released an unpublished study to CNN that directly linked the Flint water crisis to an outbreak of Legionnaires’ disease in which 12 people died and another 90 were sickened. Edwards is the independent researcher who found lead in Flint’s drinking water in 2015, at the same time officials were denying that lead was leaching into the water supply.

During the re-creation of the crisis in his engineering lab, Edwards proved that the corrosive water created an environment in which the Legionella bacteria could flourish. “What we discovered was that when the Flint River water went into the system, it released a lot of iron and removed the disinfectant from the water,” Edwards told CNN. “And in combination, those two factors – the iron as a nutrient and the disinfectant disappearing – allowed Legionella to thrive in buildings where it could not do so previously.”

A person usually catches Legionnaires’ disease – a serious type of pneumonia caused by the Legionella bacteria – by inhaling microscopic water droplets in the form of mist or vapor. The Flint outbreak of Legionnaires’ disease – one of the largest in U.S. history – is thought to be the first to originate in a drinking-water system.

Before Edwards released his results to CNN, there had never been a scientific link to the cause of the outbreak. Michigan officials allegedly stopped the Centers for Disease Control and Prevention (CDC) from coming to Flint to investigate during the first wave of the outbreak. CDC officials, who must be “invited” to investigate, said the state insisted on handling it. The state investigators never found a cause.

“Not having the world’s foremost experts on Legionella come to Flint and diagnose the cause of this outbreak was really unfortunate,” Edwards said. “Not just from the perspective of samples not being collected, but probably the CDC could have forced action earlier that could have prevented further outbreaks and deaths that actually occurred in 2015.”

An inquiry into the crisis and a criminal investigation against state and city officials are ongoing.