Your father

It is probably too obvious, but your father has experience and skilled henchmen to pull this murder trough. And he bribes several police officers, so it is unlikely that he gets punished in any way. The motive in not exactly clear. It could be revenge on the woman who left him and took you with her. Or it was fear for what she could tell the FBI. Whatever it is, you are sure this strange phone call had something to do with it.

But no matter what the motive is, you have too few evidences to accuse him of anything. And a crime lord like him is clearly not stupid enough to let hard evidence lay around like an often-used pencil. All you can do for now is hoping that you can act normally in presence of the assumed murder of your mother. But you will find something. Sooner or later.