ODESSA — Staff at the Big Stone National Wildlife Refuge in western Minnesota are investigating a turtle die-off discovered along the Minnesota River in the refuge over the weekend.

Visitors to the refuge on its auto tour route first reported spotting the dead turtles, according to Tina Shaw, public affairs specialist with the U.S. Fish and Wildlife Service in Bloomington. Law enforcement for the refuge learned of the situation on Friday and began an active investigation.

Staff members are currently collecting the turtles to determine the extent of the loss. They are estimating 100 to 200 dead turtles. Collection efforts are ongoing and that is not a final count, according to Shaw.

The dead turtles include both painted and snapping turtles of various sizes. It’s not known if other species were affected. The turtles were discovered in the river water and on the riverbank along an approximate half-mile stretch of the river.

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At this point, the cause of the die-off is not known. Samples from the turtles have been sent to a federal laboratory in Madison, Wisconsin, for analysis.

Staff at the refuge were in the process of recovering the dead turtles on Monday and hope to get a more accurate number of the toll.

Visitors to the refuge are advised not to touch the turtles. Turtles are susceptible to salmonella and contact is not advised, Shaw said.