BOSTON -- A former U.S. gangster who admitted killing 20 people was unemotional Monday when describing his work during the trial of his former partner, James "Whitey" Bulger, but he called himself heartbroken when he learned that Bulger had become an FBI informant.

Bulger, now 83, was one of the nation's most wanted fugitives for years until he was caught in 2011. He is charged in a broad racketeering indictment that accuses him of participating in 19 murders in the 1970s and '80s.

John Martorano was questioned about a string of murders he committed while he, Bulger and Stephen "The Rifleman" Flemmi were members of the Winter Hill Gang.

"They were my partners in crime, they were my best friends, they were my children's godfathers," Martorano said. He said he was stunned to learn years later that Bulger and Flemmi had been providing information to the FBI at the same time they were committing crimes.

Martorano, 72, is one of three former Bulger cohorts who co-operated with the government and agreed to testify against Bulger in return for reduced sentences.

Martarano described how he said Bulger first became involved with former FBI Agent John Connolly Jr. He said Connolly met with Bulger's brother, former Massachusetts Senate President William Bulger, in the mid-1970s to express his gratitude for being a mentor to him and to offer him help if needed.

Martorano said Whitey Bulger told him that his brother had told Connolly, "If you could keep my brother out of trouble, that would be helpful."

Martorano said that after that, Connolly helped the gang by tipping them off to investigations.

Connolly was convicted of racketeering for tipping off Bulger to an indictment, causing him to leave Boston in 1994.