Jurors hearing the murder and racketeering trial of James "Whitey" Bulger have been shown weapons including machine guns.
The prosecution alleges that Mr Bulger's gang used the firearms in crimes including shaking down smaller-time crooks.
One investigation into the "Winter Hill" gang started in 1990 by working with suspects who ran illegal gambling rackets.
Retired Colonel Thomas Foley said: "We decided that the best way to attack the organisation was through the bookmakers,"
He added that extorting bookmakers, who ran gambling rings, was an important source of revenue for Mr Bulger's gang.
Bulger, now 83, has pleaded not guilty to all charges including racketeering, money laundering and 19 murders he is accused of committing or ordering in the 1970s and 80s.
If convicted he faces the possibility of life in prison.
During the trial's opening yesterday, defense attorney JW Carney cast the accused as a mild-mannered gangster who ran highly profitable loan-sharking, gambling and drug operations.
He said Mr Bulger's former criminal associates blamed murders they had committed on Bulger in exchange for more lenient sentences.
Mr Bulger's story began in a largely Irish working-class neighbourhood.
He became one of Boston's most powerful and feared criminals and disappeared for 16 years.
He evaded arrest even as his name was listed on the FBI's "Most Wanted" list.
His story has fascinated Boston for years and large crowds of onlookers have packed into court to catch a glimpse of what is expected to be a four-month trial.
It also stood as a black mark on Boston law enforcement as Mr Bulger for years worked with FBI agents who were more focused on breaking gangs made up of ethnic Italians than criminals who shared their Irish ancestry.