Thousands of pages of emails and other documents, two Congressional hearings and multiple criminal and administrative investigations have revealed at least a dozen instances in which city, state and federal officials made crucial decisions — or failed to take effective action — that contributed to the water crisis in Flint, Mich.

Here are decisions and actions at notable junctures that led to the contamination of Flint's water supply; a delay of more than a year before residents were alerted to their exposure to lead; and elevated blood lead levels in Flint's children.

Lead has been linked to health and developmental problems, and its effects can be irreversible.