Federal prosecutors are now working with the EPA to investigate Flint’s water crisis.
The U.S. attorney’s office did not say if it’s a criminal or civil investigation.
The water emergency is linked to elevated levels of lead due to the water supply.
The city switched from Detroit’s water system to Flint River water to cut costs.
Last week, Governor Snyder apologized and Michigan’s top environmental regulator resigned.
Flint is now back on Detroit’s water system.