the NTSB chair has been going after the FAA, saying that the FAA failed to adhere to 15,000 close call warnings at Reagan National Airport between planes and choppers, failed to act, failed to move the helicopter route so that there would not be a disaster. Failed to listen to controllers who warned of a possible disaster. And she says the FAA has been dragging its feet and not cooperating in this investigation.
This is precisely why the FAA and NTSB are two separate agencies: When you're looking into why a crash happened, it often develops that the FAA wasn't doing some things it should have been doing. And, of course, if the FAA was investigating itself, it would be tempted to downplay how that happened or how big a deal it was. Since it's a totally separate organization that does investigations as the one that enforces airline safety day-to-day, though, they can just state the facts and if it hurts someone's feelings or career prospects, oh well.
It's a leftover from an earlier and more honorable time in American governance.