John Martorano, 72, took the stand against his former criminal associate James Whitey Bulger on Monday and told the jury about killing rivals in drive-by shootings as his friend became the kingpin in Boston organized crime circles.

According to Reuters Martorano is the first of Bulger's former associates to testify against him in federal court, where Bulger is being tried on charges including racketeering and 19 murders he either committed or ordered while running Boston's Winter Hill crime gang in the 1970s and 80s.

Bulger, 83, has reportedly pleaded not guilty to all charges. He faces the possibility of life in prison if convicted.

On the stand Martorano, who has spent 12 years in prison for 20 murders he confessed to, recalled several execution-style killings of Bulger rivals and people mistaken for them, in which he served as gunman while Bulger drove a backup car.

The night of December 1, 1973, when they set off to kill rival James O'Toole was different, Martorano said.

'This night here, Whitey wanted to do the driving, so he came with me,' Martorano recalled. Bulger, who served as lookout while Martorano got out of the car and gunned O'Toole down with a machine gun, had to wave off a passerby who came walking down the street.

'He chased him off with his hand, and he said, 'I'm never going to be in the car without a gun again,' Martorano said.

That night the gang members reportedly gunned down O'Toole, said Martorano, but they also shot dead at least two other people unintentionally in their efforts to murder rival gang boss Al Notarangeli, whom they shot dead in February 1974.

Martorano also told the jury that it broke his heart to learn that Bulger had served as an FBI informant. Martorano said he had named his youngest son James Stephen in honor of Bulger and another associate, Stephen Flemmi.

The FBI has extensive files of information it says Bulger provided them during the years when investigators who shared Bulger's Irish American background worked with him to take down the Italian mafia in the United States.

Bulger also denies being an informant.

Bulger's lawyer has described him as a criminal who engaged in illegal gambling, loan-sharking and drug dealing but not murder. Prosecutors instead portray him as a cold blooded killer.

Bookmaker Richard O'Brien, 84, testified that he began working with Bulger's criminal gang in the early 1970s. The arrangement was helpful to enforce collections on debts he was owed.

'When we had a problem, the best thing I had was to say, ‘Do you want to speak with someone from Winter Hill?' O'Brien testified.

O'Brien also reportedly said he had lied during grand jury testimony in 1995 when he denied that he paid protection money to Bulger out of fear of what would happen to him if he testified against Bulger and his associates.

'I wouldn't testify against those people because of the repercussions you could have,' he said.

FBI James 'Whitey' Bulger's old mugshotsGoogle Images