Whitey Bulger Trial: John Martorano, reputed hitman, takes the stand for second day in Boston Tuesday

(CBS/AP) BOSTON - Former hitman John Martorano, who admitted killing 20 people, was set to testify for a second day in the racketeering trial of James "Whitey" Bulger, as Bulger's attorneys prepared to attack his credibility.

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VIDEO: The Executioner: John Martorano speaks with 60 Minutes

Martarano took the stand Monday to testify against Bulger, a man to whom he was once so close he named his youngest son after him.

He said he was heartbroken when he learned in the late 1990s that Bulger and partner Stephen "The Rifleman" Flemmi had been working as FBI informants. That's when he decided to cooperate with prosecutors and testify against Bulger and others in exchange for a reduced sentence, he said.

Bulger's lawyers did not get a chance to question Martorano on Monday, but they are expected to challenge his credibility and the deal he got from prosecutors when they cross-examine him, possibly as early as Tuesday. Martorano served just 12 years in prison after admitting to the 20 murders. He was released in 2007.

In opening statements last week, Bulger's lead attorney, J.W. Carney Jr., told the jury that prosecutors were so desperate to get Martorano to testify that "they basically threw their hands up in the air and said, `Take anything you want."'

Speaking on CBS This Morning Tuesday, CBS News legal analyst and commentator Rikki Klieman described Martorano's testimony as "highly credible" and "ice-cold."

"This cross examination is going to be critical for J. Carney," Klieman said. "He has got to show that the government will stop at nothing to convict Whitey Bulger."

Martorano can "put Whitey in the midst" of 11 of the 19 murders he is charged with, Klieman said.

Bulger, now 83, is charged in a broad racketeering indictment that accuses him of participating in 19 murders in the 1970s and `80s.

Martorano spoke nonchalantly Monday when describing a string of murders he committed while he was a member of the notorious Winter Hill Gang in the `70s.

But he said he felt betrayed when he learned his former partners - Bulger and Stephen "The Rifleman" Flemmi - were FBI informants.

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

Martorano is the first of three former Bulger cohorts who have cooperated with prosecutors and agreed to testify against Bulger.

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

During his testimony, Martorano described what he said was Bulger's involvement in several killings, saying that while he shot someone from a car, Bulger and others would ride in a second car to intervene if anyone tried to stop the shooting.

He described the death of one victim, Alfred Notarangeli, in 1974.

Martorano said Bulger's gang decided to kill Notarangeli to help the Italian Mafia in Boston, a sometime rival, whose leadership said Notarangeli had killed one of their agents and was a "loose cannon."

On March 8, 1973, Martorano said, he drove in the lead car while Bulger followed, both tailing a Mercedes they believed was driven by Notarangeli.

"We pulled guns and we were shooting at it," Martorano said, referring to himself and another member of the gang.

They later learned that it was not Notarangeli in the car, but instead a man named Michael Milano, who was shot to death.

Martarano said they continued to chase Notarangeli and ended up killing him and his brother, Joseph Notarangeli.

Bulger is charged in the killings of both brothers, as well as Milano's killing.

Flemmi and another Bulger cohort, Kevin Weeks, are also expected to be key prosecution witnesses.

Complete coverage of the Whitey Bulger case on Crimesider