More than 100 turtles were found dead in a western Minnesota wildlife refuge, and federal officials are so far unable to say what's behind the rare die-off.

Law enforcement officials at Big Stone National Wildlife Refuge were alerted to the dead painted and snapping turtles on April 30, U.S. Fish and Wildlife Service (USFWS) spokeswoman Tina Shaw said Thursday.

Refuge staff members have estimated the extent of the die-off at anywhere from 100 to 200 turtles, "but collection efforts may still be ongoing at this time, so this is not a final count," Shaw said.

Regardless, she said, this "is something I've never seen before in the 10-plus years I've been based here in the Midwest. Refuge staff are still looking into this situation, and there's a lot that we don't know at this point."

The turtles of various sizes were discovered in the water and on a half-mile stretch of riverbank of the Minnesota River, Shaw said.

Some of the turtles have been sent for testing to the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis.

The painted turtle is the region's most common of the species. Females grow to 10 inches in length, while males top out at 7 inches.

The snapping turtle is Minnesota's largest of the species. Adults measure from 8 to 14 inches in length and weigh from 10 to 35 pounds.

Big Stone National Wildlife Refuge, established in 1975, sits just shy of the South Dakota border with 11,586 acres of tallgrass prairie, wetlands, granite outcrops and river woodlands. Eleven miles of the Minnesota River flow through the refuge.

Paul Walsh • 612-673-4482