Today in school I heard this (when we had Criminal Law):

Somewhere in history the Commissaris of Police (Head of Police) in Boston infiltrated in the Irish Mafia of Boston. This was just because of the fact that he had sympathy for them and not the Sicilian Mafia. When he infiltrated he got very adequate files of murders/tips etc on the Sicilian Mafia. When time was evolving many Italian gangster were arrested and prosecuted. The Irish Mafia grew, because the Italians lost power. But to get this result the Head of Police didn't tell other police members about 18 murders he knew of committed by Irish Mafia...

Hope it's clear what I'm trying to tell, but can someone tell me more about this?

Maybe Turnbull???