From what we know so far, 3 cops (Raglan, McCallister and an unknown) is involved 19 years ago with kidnapping Mafiaso for ransom. That third unknown cop, now rich from the ransom money and a few smart investments over the past 19 years, can afford to hire assassins to cover up his past crimes.
So far out of everyone we know on Castle, both Montgomery and possibly the Mayor fits the bill. Personally, I feel Montgomery has that "omgicantbelieveitshim!" shock factor.
Then again, McCallister said Beckette has "woken the dragon". Montgomery doesn't fit the profile of a dragon. So very likely, that third unknown cop could be a totally unknown third party.
