I'm surprised Hill didn't claim at one time Al Capones ghost was responsible for taking out Tommy. A silenced .38 Magnum? Ha, ok.

If one were to read Joe Ianuzzi's book, (as stated above) Agro seems to have been the person most likely responsible. There's only a couple of paragraphs where they discuss the DeSimone brothers getting killed. This was before Pesci's portrayal of an amalgam of Tommy and Goodfellas. Of course someone like Hill would claim one of the most famous/infamous gangsters in history would somehow be involved.