"....it's not clear to me why FITCH and MOODY's are also not being investigated..."
Good point.
All THREE ratings firms are EQUALLY guilty...and have been for a LONG time. They all should be investigated. Then again, we don't know for certain if the rest of the lot is being investigated anyway....so who knows?
While the investigation has been underway well before the ratings downgrade, today's announcement will no doubt give a TON of ammunition to all those crazed conspiracy theorists out there (claiming the Government is "investigating" them for downgrading our worthless trash).
This is either a classic case of bad timing, or Standard and Poors caught wind the Justice Department was planning on making the investigation official and decided to issue the downgrade.