Black Hills, by Nora Roberts

Lil Chance met Cooper Sullivan one summer when he came from New York to spend the summer with his grandparents in South Dakota, when she was just 11. They became good friends and saw each other almost every year until she was age 20. By then there was a romance between them, but Cooper broke it off while trying to finish school and make his way for himself. But not before Lil found a body in the Black Hills. Twelve years later, Lil has created The Chance Wildlife Refuge, where cougars, tigers and other animals are kept and cared for. Cooper comes back to live with and help his grandparents and he and Lil start to see each other around again. Someone kills a cougar that Lil was tracking, and other things start to happen, among them a trail hiker being killed and found in the river. Cooper, being an ex-private investigator, digs around & they narrow their search for the killer down to a guy who once volunteered at the refuge, named Ethan. Ethan believes he is Sioux Indian and is after Lil. With the help of Coop, and a cougar named Baby, the hunt comes to an end.