PENFIELD, Pa. — In the heart of Pennsylvania elk country, Eric McCarthy and his client, Don Reichel, got up before sunrise to scour the forest floor for so-called “brown gold” — a rack of freshly shed antlers to add to Reichel’s collection back home. One hill over, a team of FBI agents was also hunting for gold. The metallic yellow kind. The FBI’s highly unusual search for buried Civil War-era treasure more than five years ago set in motion a dispute over what, if anything, the agency unearthed and an ongoing legal battle over key records. There’s so much intrigue...