Worries over bacteria led to the fateful decision not to apply anti-corrosive chemicals when Flint, Michigan, began drawing water from the Flint River, an email written by the city’s former public works director shows.

The failure to deploy corrosion controls after the city switched to the river from the Detroit municipal system in April 2014 has been widely considered a catastrophic mistake that enabled lead to leach from aging pipes and reach some homes. The Michigan Department of Environmental Quality has acknowledged instructing Flint not to use corrosion controls, based on a misreading of federal regulations.

In a Sept. 3 email to numerous state and local officials, then-public works director Howard Croft said a different concern had been identified.

As the city water plant was designed to begin treating river water, “optimization for lead was addressed and discussed” with an engineering firm and the DEQ, Croft wrote. “It was determined that having more data was advisable prior to the commitment of a specific optimization method. Most chemicals used in this process are phosphate based and phosphate can be a ‘food’ for bacteria.”