The Michigan Department of Environmental Quality is admitting a mistake made while working with Flint on the city’s water crisis.

The state says the Office of Drinking Water and Municipal Assistance made an error in testing by following protocols for a city with fewer than 50,000 residents instead of for a city with 100,000 or more.

DEQ Director Dan Wyant says in a statement some department staff lacked experience with corrosion control.

“Simply said we do have to, again, acknowledge our mistake in this. The governor will call for an after-action review, and that review will look at local decision makers, plant operators our own actions on EPA and determine whether others should have taken additional action,” Wyant says.

The City of Flint will return to Detroit’s water system after months of complaints about water quality and lead exposure.