For decades, treasure hunters in Pennsylvania have suspected that there is a trove of Civil War gold lost in a rural forest in the northwestern part of the state.

But the mystery about where it is hidden, or if it even exists, has recently deepened.

Last week, F.B.I. representatives showed up at a site in Dents Run, Elk County, an area known for its seasonal elk viewing activities that feed the economy of nearby Benezette Township.

The agency, in a statement on Monday, said very little about the mission aside from describing its work as a “court-authorized excavation” at Dents Run that ended on March 14. Its conclusion: “Nothing was found.”

An F.B.I. spokeswoman, Carrie Adamowski, declined to comment further.

The sudden and apparently secretive appearance of federal investigators at the site has deepened the mystery over the fate of the gold bars that has persisted for more than a century, despite the efforts and hopes of treasure hunters, the study of historians and the years of scrutiny by local news media.