The state of Michigan is suing Flint, alleging that the city council's failure to approve a recommendation to buy water long term from a Detroit-area system is endangering a public already troubled by a lead-tainted water crisis.
The lawsuit was filed Wednesday in federal court.
The state Department of Environmental Quality had previously threatened legal action if the council did not approve Mayor Karen Weaver's recommendation or a reasonable alternative by Monday.
The council instead approved a short-term extension of its contract with the Great Lakes Water Authority, against the wishes of Mayor Karen Weaver, who is backing a 30-year contract.
State and federal officials do not want Flint to change water sources a third time after a fateful 2014 switch resulted in lead contamination and other issues while the city was under state management.