DA informed me through email.

Among many replies I received from USCIS to my inquires about my case during the past 2 years, sometimes they mentioned "security check" as the reason for delay, others just said my case was "under review", or "waiting to be assigned to an officer". So I couldn't tell when the NC started and I did not know if it was clear when I filed the WOM case. I did not even list FBI as one of the defendents at all. So until the DA told me in May I had no idea about it.