John "No Nose" DiFronzo Mentioned in Chicago Mob Trial

The Chicago Sun-Times reports:

In a straightforward tone, a federal prosecutor today for the first time laid out details in the deaths of 18 men and women allegedly slain by Outfit killers as part of opening statements in the historic Family Secrets case.

Among the highlights was the first public allegation that John DiFronzo, a reputed high-ranking Chicago mob boss, was involved in the 1986 murders of Anthony and Michael Spilotro. DiFronzo is not among the five men charged in the case, and the prosecution did not address why he has not been charged.

Why wasn't DiFronzo indicted?