Federal prosecutors say they're working with the U.S. Environmental Protection Agency on an investigation into problems with lead in Flint's water supply.

U.S. Attorney's spokeswoman Gina Balaya said in an email Tuesday that the investigation is "an effort to address the concerns of Flint residents."

Balaya couldn't say whether it is a criminal or civil investigation.

The city switched from Detroit's water system to Flint River water in a cost-cutting move in 2014, while under state financial management.

Residents complained about the water's taste, smell and appearance and children were found to have elevated levels of lead due to the water supply.

Last week, Gov. Rick Snyder apologized and Michigan's top environmental regulator resigned.

Flint returned to Detroit's water system in October.