Watch CBSN Live

Whitey Bulger Trial: Hitman testifies reputed mobster "broke my heart" after allegedly acting as informant

John Martorano, a former admitted hit man for Boston mob figure James "Whitey" Bulger. CBS News

BOSTON (CBS/AP) -- An ex-gangster who admitted killing 20 people was unemotional Monday when describing his line of work at the trial of his former partner, James "Whitey" Bulger. But he called himself heartbroken when he learned that Bulger had become an FBI informant.

PICTURES: Guns and Money: Whitey Bulger evidence

VIDEO: The Executioner: John Martorano speaks with 60 Minutes

John Martorano gave short, matter-of-fact answers when questioned by a prosecutor about a string of murders he committed while he, Bulger and Stephen "The Rifleman" Flemmi were members of the Winter Hill Gang.

The only flash of sentiment came early in his testimony, when he was asked to describe his relationship with Bulger and Flemmi.

"They were my partners in crime, they were my best friends, they were my children's godfathers," Martorano said. He said he named his youngest son James Stephen after Bulger and Flemmi.

Martorano 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 for the gang.

"After I heard that they were informants, it sort of broke my heart," he said.

Martorano, 72, served 12 years in prison after striking a cooperation deal with prosecutors.

Bulger, now 83, is charged in a broad racketeering indictment that accuses him of participating in 19 murders in the 1970s and '80s. He is also charged with extorting bookmakers, drug dealers and others running illegal businesses.

Martorano testified Monday that he fatally shot Alfred "Indian Al" Notarangeli from a car in 1974 as Bulger rode behind him in a second car, reported. He was the first witness to tie Bulger to a murder, according to the report.

Martorano also described other murders he committed, including two men he said he shot to protect his brother from being prosecuted for the 1964 murder of a waitress, reported.

Martorano is a key witness for the prosecution. But Bulger's lawyer, J.W. Carney Jr., attacked his credibility in opening statements, citing his "extraordinary" plea deal with prosecutors.

"The federal government was so desperate to have John Martorano testify ... they basically put their hands up in the air and said take anything you want," Carney said.

Complete coverage of the Whitey Bulger case on Crimesider