The gold currently in Fort Knox came from the melting of Depression-era gold coins, from lend-lease arrangements in War II, and from government operations under the gold standard.”
Most of those gold coins were likely the coins confiscated from private owners by the US government following Roosevelt’s Executive Order 6102 which outlawed the private ownership of gold.
Moreover, private banks and the central bank held gold in the form of coins for dollar holders who, prior to confiscation, would occasionally present US dollars for redemption in gold. This is, in part, the gold in Fort Knox that that Wood classifies as gold held for “government operations under the gold standard.”
Wood, W. C. (1994). The Gold at Fort Knox. The Journal of Economic Education, 25(4), 343–348. https://doi.org/10.1080/00220485.1994.10844844