Here me out on this one, The government of the USA has the right to withhold information especially when it involves a threat to national security disclosing such information or confirming that their is Gold in Fort Knox will make that place a known target for criminals or even terrorist who really wants the economy of USA collapse, saying that there is no Gold in Fort Knox will definitely give a massive panic but the most brilliant way to avoid this is stating that the gold is on an undisclosed place well guarded by the government.