They can't. The big bad government, and wheels of legal bureaucracy, has told them to remain mum.
The "documents" will be something cryptic (names, dates, events) that I'm sure they can come up with. And as for the treasure part of the story ingredient ? I'm sure they can come up with a reason why it's not on the records. Like: So that enemy soldiers couldn't/wouldn't know, and thus risk theft.
Or they'll point you to the legend itself, as the requisite "documents". Anytime you ask for proof, all they need to do is point you back to the legend itself, and presto: There's your "proof". And then it just becomes a game of wack-a-mole to debate how far back the legend's origins goes. And to them, the mere mention of a name in history ("joe blow" or "president Lincoln") is the proof they're seeking.