Well, that's stupid. From an FBI/Homeland Security standpoint, it's stupid to announce that they're now on the case for two reasons. #1, it lets the hackers know what's being used against them and gives them a chance to respond, and #2 it entirely swaps the results from a PR standpoint. If the FBI had started working on the case and never told anybody, the possible results would have been: Don't catch the guys and nobody knows you failed because no one knew you were looking for them or catch the guys and announce their capture to great applause. Now, instead, the outcomes are: Don't catch the guys and look incompetent or catch them and look like you were only doing your job. Sony is the only party which benefits from the FBI/HS people coming out and telling everyone that they're on it.
EDIT: To clarify, the hackers now know they are being sought. If they're in the US and have passports, they can use any more they've gain from selling CCNs to simply leave before they get caught. Once they're in another country, it'd be a lot harder to get them back, especially if they pick the country very well. An extreme example, but there was no benefit to making the announcement and it opens up this possibility.