The Costello Gang was founded in 1960 by Francis Costello, who was an Irish-American citizen. Costello and his right-hand man Arnold French hired enforcers such as Timothy Delahunt and Patrick Fitzgibbons, and they took over E Street, working an auto repair shop and the Park Luncheonette as rackets. Costello had police protection since he was an occasional FBI informant, ratting out members of other gangs, but never his own men.
Costello was racist against African-Americans, beating them down in the 1970s. His gang also fought against the Providence Mob, who were part of the Italian Mafia. He did business with the Triads in Boston, selling them microprocessors in 2006, which turned out to be fake, and he stole the whole suitcase of money.
The gang was brought under government attention in 2006 after Myles Kennefick stole some of the microprocessors, before Costello murdered him, because he was a liability. Costello made a deal with the Chinese government, so the FBI and Massachusetts State Police attempted to stop him. They tracked him down, since they had a rat in his organization, Billy Costigan, while Costello had a rat in the police, Colin Sullivan. Sullivan killed Costello in the Sheffield Warehouse shootout, after he found out that he was a member of the FBI, so that he could not rat him out. Sullivan killed Costigan at a shootout later on, to preserve his identity as a police officer, but he was killed as well. Every member of Costello's Gang had died by that point, most of them at the warehouse.
Boss: Francis Costello
Underboss: Arnold French
Lieutenant: Patrick Fitzgibbons
Soldiers: Timothy Delahunt and Billy Costigan
Associates: Jimmy Bags and Colin Sullivan