"Nobody works here anymore" is a classic excuse used in information requests like this.
These letters the DoD sent back sound exactly like the letters I read every day as a civil litigator during discovery for cases. The DoD is basically telling this inspector to fuck off and stop bothering us.
That's not to say the Pentagon has no idea where the money went. They are just not cooperating with the investigation. The DoD is saying "we fired those guys, you go find them yourself."
And it sounds like the Special Inspector General has a pretty good idea why the natural gas station was so expensive. The organization in charge didn't do a feasibility study. Then spent millions of dollars building a station when it wasn't a good idea.
Nobody stole the money. They just squandered it on a gas station to nowhere.
Edit: Unsurprisingly both the inspector general and the DoD Deputy Under Secretary are both trained lawyers. The inspector general was even a civil litigator up until a few years ago.