The lawyers for James Whitey Bulger sought to raise doubts about the accuracy of a 700-page informant file that a now-disgraced FBI agent had kept on the accused mob boss.
The file, a key piece of evidence in the trial, was developed through the 1970s and '80s when Bulger is accused of murdering or ordering the murder of 19 people.
According to Reuters on Tuesday jurors heard extensive testimony based on the file about meetings during which Bulger provided tips on his gangland rivals to his FBI handlers, including John Connolly.
Bulger's attorneys cross examined the FBI Special Agent James Marra, who headed the Justice Department probe that lead to Connolly's conviction on murder and racketeering charges on how the bureau protected 'top echelon' informants such as Bulger.
But Bulger has denied ever being an FBI informant, arguing that he paid Connolly for information but never provided tips of his own.
The 83-year-old, who has pleaded not guilty to all charges, faces the possibility of life in prison if convicted.
'Can you confirm firsthand that Bulger gave any of that information?' Bulger attorney Henry Brennan asked Marra.
'Firsthand? No,' the agent replied.
'The federal government went out of their way on many occasions to protect their informants, didn't they?' Brennan later asked.
Jurors heard how Connolly had set up alerts in the Justice Department computer systems that ensured he was tipped off whenever another law enforcement agent ran a background check on Bulger.
Connolly, who shared Bulger's Irish background, allegedly turned a blind eye to crimes which Bulger is accused of committing in exchange for information on the doings of the Italian Mafia. The Mafia at the time was the top priority of the US Justice Department.
In court on Tuesday Marra acknowledged that agents such as Connolly received financial incentives from the FBI to develop high-level informants such as Bulger.
'I don't know if it was an enormous incentive, but the agents were encouraged to cultivate informants,' Marra said.
Bulger's attorneys have argued that Connolly made up at least some of the information in Bulger's file to justify his frequent meetings with the gangster.
Meanwhile prosecutors have openly scoffed at the idea that Bulger was not an informant, noting that he met with several other FBI agents and supervisors in addition to Connolly.