FBI background checking delayed for citizenship

Hi all, (sorry to type in English, my chinese software has some problem).

I went to citizenship interviewed today, it passed pretty easily, but then the officer told me my FBI background checking is not passed yet. He said it is rare and random and can take up to a year. Normally FBI should have already be done by the time of the citizenship interview! My bad luck I guess.

Does anyone has any or has heard of any experience like this? What should be my expectation? Will it take very long?

thanks!