State officials are investigating a fish kill in northeast Iowa.

According to a press release, specialists with the Iowa Department of Natural Resources discovered the kill in Dubuque County along a 10-mile stretch of the Little Maquoketa River north of Epworth downstream to Durango. DNR officials said they found thousands of dead fish, mostly rough fish and minnows.

The source of the fish kill has not yet been identified. DNR officials said a pollutant could not be identified from preliminary field tests.

DNR staff have taken water samples for testing. State officials plan to continue monitoring the river as they look for the pollutant source.