Based on two years of reporting, a new FRONTLINE documentary called "Flint's Deadly Water" reveals how a public health disaster that’s become known for the lead poisoning of thousands of children also spawned one of the largest outbreaks of Legionnaires disease in U.S. history.

“Most people outside of Flint look at the lead issue as the main issue,” Flint City Council Member Eric Mays tells FRONTLINE in the documentary. “But the killer has been Legionnaires.’ … I still don’t think that they want people outside of Flint to know.”

The official tally is that 12 people died and 90 people were sickened due to a Legionnaires outbreak that happened right after the water supply in Flint was changed in order to prepare for a new pipeline the state said would save the city money. But FRONTLINE reporters on the ground say the data and the people of Flint bear witness to a very different story: The outbreak continued for more than a year before state officials alerted the public.

"Just a couple of weeks after the switch, we start to see cases pop up," said Abby Ellis, the director of "Flint's Deadly Water," on Boston Public Radio on Tuesday. "What's pretty important to note is that the outbreak of Legionnaires disease that started right after the switch and persisted, state officials knew about this outbreak ... the officials who knew there was a bacteria in the water that could be killing people, were the same officials in many cases who kept the city on the river for another six months."

"Flint's Deadly Water"premieres on PBS Sept. 10 at 10 p.m.