In July, Flint sent residents a letter saying it was “pleased to report” the “water is safe.”

But officials’ efforts to soothe residents about other contaminants seemed to overshadow the growing signs of trouble about lead.

By March 2015, with residents turning up at public events bearing bottles of murky water, the City Council voted to “do all things necessary” to reconnect to Detroit’s water system. But the state-appointed emergency manager, Gerald Ambrose, said no. He repeated the official mantra: The water meets state and federal standards. And he noted, once more, that Detroit water was among the most costly in the state.

“Water from Detroit is no safer than water from Flint,” Mr. Ambrose said.

Corrosion Control Failure

Behind the scenes, though, officials seemed far less sure.

By the end of February, Miguel Del Toral, the E.P.A. regulations manager who had learned of high lead content in one Flint resident’s water, was raising a fundamental question with his state and federal colleagues: What was Flint using to treat the river water to avoid corrosion?

“They are required to have O.C.C.T. in place which is why I was asking what they were using,” he wrote in an email on Feb. 27, using the initials for “optimal corrosion control treatment.”

Surely, the assumption was, the city was adding a chemical to the water to coat its aging pipes and prevent corrosion, since controlling corrosion is required by a federal rule governing lead and copper. The water that Flint had drawn for years via Detroit from Lake Huron had been treated with orthophosphate, a common anti-corrosion additive. And Flint River water is naturally even harder and more corrosive, experts say, than the water the city was buying from Detroit.

An official from Michigan’s Department of Environmental Quality answered Mr. Del Toral’s inquiry the same day: Flint has “an optimized corrosion control program.” But less than two months later, the state said it had been wrong. There actually was no treatment in place in Flint to stop corrosion, a timeline of events provided by the state now shows.