The drowning man / Margaret Coel.

Enhanced descriptions from Syndetics:

In Margaret Coel's latest Wind River Reservation mystery, Arapaho attorney Vicky Holden and Father John O'Malley find themselves immersed in the dark underbelly of the illegal market for Indian relics. The Arapahos call it "The Drowning Man"-an ancient petroglyph depicting the haunting image of a human figure struggling under water. A priceless artifact to the tribe, the sacred object is worth a quarter of a million dollars to the thieves who took it. After receiving the ransom demand, Father John is determined to uncover the identity of the culprits and recover the petroglyph. The theft is nearly identical to an unsolved seven-year-old case involving another stolen petroglyph-and manslaughter. Vicky joins Father John to piece together the events of the past seven years. But their quest will put them in the path of a relentless killer who will stop at nothing to remain unknown.