An Ex-FBI agent was called as the first witness by James "Whitey" Bulger's lawyers as they began to present their defence.
Robert Fitzpatrick testified that Mr Bulger kept dodging his questions, and the agent quickly determined that he was not a cooperative source.
It was unclear whether the former head of Boston's Winter Hill crime gang would testify in his own defence on charges linked to 19 murders he is accused of committing or ordering in the 1970s and 1980s.
Mr Bulger, 83, has pleaded not guilty to all charges, though his lawyers have admitted their client was a drug dealer, extortionist, loan shark and "organised criminal."
But what his lawyers have argued most vociferously is that Mr Bulger was not, as prosecutors contend, an FBI informant.
"At one point, he even said he was not an informant," Mr Fitzpatrick said. "At that point, I made a mental reservation, 'What am I doing here? What's going on here?'"
Mr Bulger, who inspired the character played by Jack Nicholson in the 2006 Academy Award-winning film The Departed, contends he paid corrupt FBI agents for information but provided none of his own.
Becoming an informant, a "rat" in the parlance of Boston's Irish gangs was a severe breach of the mob code.
During the first seven weeks of the trial, prosecution witnesses testified that Mr Bulger killed several people because he was convinced they were talking, or might talk, to authorities.
Early in the trial, Mr Bulger shouted curses at former associate Kevin Weeks when he testified that Bulger had been an informant.
Mr Fitzpatrick, 73, said he decided to meet with Mr Bulger in early 1981.
He had just been named assistant special agent in charge of the Boston bureau of the FBI and ordered to look into the office's relationship with "Whitey" Bulger.
FBI agent John Connolly and his supervisor John Morris had developed Mr Bulger as their office's most highly placed informant.
He was viewed as a key source of information that they could use against the Italian Mafia, then a top FBI target.
But other law enforcement agencies, including the Massachusetts State Police, had begun to complain about what they said was preferential treatment for the gang boss.
Mr Fitzpatrick said Mr Bulger told him that he was the head of the Winter Hill gang, which made the agent even more wary.
"You can't have the head of a gang as an informant, because then you're validating the gang, you're actually part of the management process if you will," Mr Fitzpatrick recalled.
He told the jury he returned to the office and told his boss it was time to close off "Whitey"Bulger as an informant, allowing the FBI to turn its investigative forces on the gang boss.
Mr Bulger rose to power with the help of Mr Connolly, who shared his Irish ethnicity and South Boston upbringing and turned a blind eye to the Winter Hill gang's crimes in exchange for information the FBI could use against the Italian Mafia.
Mr Bulger fled Boston in 1994 after a tip off.
Agents caught up with him in June 2011, living in a seaside apartment in Santa Monica, California, with about 30 guns and more than $800,000 in cash.