I think they already know the guys behind the website. From an article [1] mentioned by [2] :
Documents obtained by TorrentFreak dated September reveal the MPA, acting through legal representatives, attempting to pressure individuals who they believe are important at Nyaa and could have the ability to shut the site down.
...
While emailed threats are still a common anti-piracy strategy, we are informed that at least two of the individuals were personally served with legal documents at their homes. Others were served with similar documents via regular mail.
[1] : https://torrentfreak.com/mpa-lawyers-are-trying-to-shut-down...