Thats strange, though if they hold it would probably be for some investigative purposes and not really for cashing except the government and the central bank know about it. But this is seeming ridiculous, maybe they use it to sponsor their spies on the darkweb
It's probably, only a rumor. FBI can't have that much bitcoins. But I'm sure they seized important amount of bitcoin after they arrested btce officials and took their machines including wallets.