My guess is that the leakers aren't tied to the investigation at all, but going by hearsay. Hard to stop a leak of that kind. But to the WaPost, a janitor who once worked at the Pentagon's chow hall is a reliable source and a source "close to the investigation"
I think the leakers are being authorized by the Pentagon to stretch the revelations in this story out over a matter of weeks, which is exactly what is happening, to soften the reaction to the details when they are officially released.
A janitor would be worthless as a source if what he says could not be believed or verified by another source. (Notice that most of these sources are saying the same things)