Talk to your parents and everyone in your parents friend group to try to regain contact with A’s victims as well as anyone that witnessed any SA. Give those contacts and stories to the police and also give the contacts of others that often interact(ed) with A, since many of them may have never spoken up about it.
This is a really important situation and what you do now can prevent a life’s worth of trauma. At the same time don’t beat yourself up about it or feel guilty about it either, it’s your fault and sometimes nothing can be done, especially if many aren’t willing to cooperate with authorities.