User blog comment:Halo1081/So About Korosensei's Past.../@comment-4524007-20150511133441/@comment-5707992-20150512010158

Yanagisawa's lab was undocumented. They stated this at the beginning of the arc. There was no way the governement could've known unless it was documented, but since it wasn't, they had/have no way of knowing unless it came straight from the horse's mouth.