It's not hard to believe Frederick Silva doesn't know who specifically killed the Scoppa brothers. The informant who worked under Sal Scoppa said the hit was arranged by Vittorio Mirarchi and that a black from Toronto killed Sal Scoppa.

You hesitate to trust Frederick Silva because a lot of the info he's given on these murders place the blame on dead people but so far a lot of his info that've we've learned about has been consistently with other sources. It's not just dead people he's placed the blame on either.