More than two-dozen witnesses were interviewed by the prosecution team, this person said.

One of the more important pieces of evidence, this person said, was how city and state officials submitted documentation related to the federal lead and copper rule, which governs acceptable levels of those toxins in drinking water. The person familiar with the matter said that some officials who worked on and submitted these reports included information they knew to be incorrect.

Flint's water became contaminated with lead in April 2014 when the city, while under the control of a state-appointed emergency manager, switched its drinking water source from Lake Huron water treated by the Detroit water system to Flint River water treated at the Flint Water Treatment Plant. Michigan Department of Environmental Quality officials have acknowledged a disastrous mistake when they failed to require the city to add corrosion-control chemicals as part of the treatment process.

The corrosive water caused lead to leach from pipes, joints and fixtures. Although Flint reconnected to Detroit water in October, after state officials acknowledged the lead-poisoning problem after months of denials, the risk remains because of damage to the water infrastructure system.

Officials also are exploring possible links between the river water and the Legionnaires outbreak.

The state and city are now treating the pipes with higher levels of phosphates in an effort to build up a protective coating that will prevent lead from further leaching. Having more water flowing in the system would help that process, and that's one reason Snyder and other state officials want Flint residents to start using their taps again.