Federal US investigators are now looking into the contamination of Flint, Michigan's water supply as a state of emergency continues.
The FBI has joined a criminal investigation into lead contaminated drinking water in Flint, Michigan, exploring whether any laws were broken in a crisis that has captured international attention.
Federal prosecutors in Michigan are working with an investigative team that includes the FBI, the US Postal Inspection Service, the US Environmental Protection Agency's Office of Inspector General, and the EPA's Criminal Investigation Division, a spokeswoman for the US Attorney's Office in Detroit said.
A Federal Bureau of Investigation spokeswoman said the agency will determine if federal laws were broken, but declined further comment. EPA Administrator Gina McCarthy is meeting with officials and community leaders in Flint on Tuesday.
The city, about 100km northwest of Detroit, was under the control of a state-appointed emergency manager when it switched the source of its tap water from Detroit's system to the Flint River in April 2014.
Flint switched back last October after tests found high levels of lead in blood samples taken from children. The more corrosive water from the river leached more lead from the city pipes than Detroit water did. Lead is a toxic agent that can damage the tissues of the nervous system.
Michigan Governor Rick Snyder, who extended a state of emergency in Flint until April 14, has repeatedly apologised for the state's poor handling of the matter.
"It's important to look at missteps at all three levels of government - local, state and federal - so such a crisis doesn't occur again," said Dave Murray, a spokesman for Snyder.
Peter Henning, a law professor at Wayne State University in Detroit and a former federal prosecutor, said there is limited ability to seek criminal charges under US environmental laws. Prosecutors would need to find something egregious like a knowingly false statement.
"You need a lie," he said. "You need something that is false to build a case."
Simply failing to recognise the seriousness of the situation would not rise to that level, Henning said.
In Washington, senators Gary Peters and Debbie Stabenow, Democrats from Michigan, pushed for $US600 million ($A851.49 million) in aid, mostly in federal funds, to help Flint replace pipes and provide health care.