The Obama administration is stepping up efforts to make sure state regulatory agencies are avoiding more cases similar to Flint, Michigan, where the drinking water supply has been contaminated with lead.

The Environmental Protection Agency sent letters Monday asking states to determine within 30 days that they are using correct procedures for treating and sampling water. Of special concern is treatment to prevent corrosion that would enable lead and copper to leach into tap water, which is what occurred in Flint.

Michigan officials did not require Flint to add anti-corrosive chemicals when the city began drawing water from the Flint River in 2014 after switching from Detroit's water system.

EPA also wants states to make sampling results and other information public to restore confidence in the nation's drinking water.