The fictional Costello's FBI links is definitely based on Whitey Bulger. He played the FBI in real life like a violin.

Higgins wrote a book based on it called, End of Day. Not one of his better books but not bad. Black Mass is the book to read about regarding Bulger and the FBI. It was written by (i think) a pair of Boston Globe journalists.

You're right, though ... that was Bulger (as regards the FBI link) in the fictional Costello.

Except, of course, Whitey walked away ...

And they want the FBI to find Ossama Bin Laden? That's pretty funny.