CLINTON, Iowa (AP) — The remains of a trumpeter swan are being analyzed in hopes of solving the mystery of what killed 32 of the rare birds in western Clinton County.

The Quad-City Times reports a hunter found remains of the birds on Jan. 30 in a privately owned wetland area. Most of the birds had been scavenged but at least one full carcass remained and it along with partial remains were sent to the Veterinary Diagnostic Laboratory at Iowa State University.

Mark Roberts, of Clinton County Conservation, called the discovery “shocking.”

Trumpeter swans, North America’s largest waterfowl species, were once on the brink of extinction but thanks to a decades-long effort now have an estimated population of more than 63,000 adult birds.