Bingo! Straight out of the mole-hunter's handbook. This should have been done months, if not years ago, and should be SOP going forward.
Individual briefings are not difficult to arrange, and each one of them should be "laced" with a nice, unique tidbit that would tempt a leaker to spill it. If it comes out, proceed to do a couple of *completely* ficticious briefings (with some really wild stuff in them) with the suspect, using different briefers (and now including FBI agents) and see if the bogus info comes out.
If it does, then haul the rat bastid up on charges, and make damn sure it's all over the press.
Problem solved!
Imal