Seems like the most likely suspect---Tommy Agro killed Tommy since before GoodFellas and Pesci's portrayal, there wasn't much info out there. In Joe "Dogs" Ianuzzi's book there was just one page dedicated to discussing T.A. killing DeSimone and his brother, who was a suspected or confirmed rat. He joked about killing the other DeSimone brother to complete the trifecta. This was before GoodFellas. After that all of the sudden Tommy became a bit of bigger deal and of course who better to say killed him in your book than Gotti himself after he had passed away in prison.