With defendant James "Whitey" Bulger sitting less than 8 feet away and staring straight ahead, a star government witness in the case against Bulger explained Monday why he turned against the man he long counted among "my partners in crime, my best friends."

Aging mobster John Martorano said he learned Bulger was an FBI informant, the worst thing you could be in "Southie," a part of Boston that prided itself on absolute loyalty.

"It broke my heart. It broke all loyalties," said Martorano, his testimony laced with what sounded like sadness.

Late Monday night, Bulger's defense filed a motion requesting that the court inform the jury that Martorano is a cooperating government witness and is testifying in exchange for a lesser sentence on murder charges. Martorano pleaded guilty to 10 murders in 1999 and later admitted to 20, so defense attorney J.W. Carney said the jury should be told to consider Martorano's testimony with caution.

Prosecutor Fred Wyshak responded Tuesday that the defense was "playing games" and said issuing a cautionary instruction mid-testimony would "send a very bad message to the jury."

Judge Denise Casper did not rule on the motion but said she would consider issuing instructions regarding nonspecific witnesses when the jury is read the final charges.

Bulger is charged in the deaths of 19 people during the nearly two decades that federal prosecutors say he was the head of the Irish mob in Boston.

For three decades, Martorano said, he was tight with Bulger and Bulger's partner, Steven Flemmi.

"They were my partners in crime, my best friends, my children's godfathers," said the 72-year-old divorced mobster who now lives on Social Security and who sold his life story in hopes of getting it made into a movie.

The men were so close, Martorano even named his youngest son "James Steven" after his two friends.

In a flat, emotionless voice, former hitman Martorano began to describe some of the murders he said he committed, several of them, he testified, with Bulger along.

Martorano said he was the trigger man while Bulger was backup to ensure the "hit" was successful.

In one case, Bulger used his car to block a target coming out a side street. Martorano, who said he was firing out the back window from another car during the incident, testified that Bulger "was afraid he was going to get shot because tracer bullets were coming out of the machine gun and flying over his head."

"Was he angry?" asked Wyshak, the prosecutor.

"No. He was joking about it," replied Martorano.

Martorano also described the killings of two brothers after one of them had angered Bulger's gang.

One brother was tracked down at a restaurant in Medford, Massachusetts, and "I shot him in the heart," Martorano said.

When the other brother was killed later, Bulger and his gang stashed the body in the trunk of a car then drove the car south and dropped it at a housing project in South Carolina where, as he had expected, "a few kids stole the car," Martorano said.

Under a plea deal, Martorano was sentenced to 14 years in prison in exchange for fully testifying against Bulger. He served 12 years and was released in 2007.

His testimony also was crucial against corrupt FBI agent John Connolly, who was found guilty in federal court on racketeering and obstruction-of-justice charges, among others.

Connolly served 10 years in federal prison and is now doing an additional 40 years in state prison after being found guilty for his role in the murder of Florida businessman and former World Jai Alai owner John Callahan.

Flemmi, Bulger's partner, is serving life terms without parole but avoided a possible death sentence by cooperating with federal authorities.

The connection between Bulger and Connolly also was the focus of Martorano's testimony Monday.

Martorano said Bulger grew up in the same Boston housing project as Connolly, who looked up to both Bulger and his brother William, who rose to serve as state Senate president in Massachusetts.

According to Martorano's testimony, when Connolly returned to Boston as an FBI agent, he thanked "Billy" Bulger for "keeping me honest" and told him "If there's anything I can do ..."

Billy Bulger said, "If you could keep my brother out of trouble, that would be a great help," according to Martorano.

Martorano testified that he, Whitey Bulger and the others agreed that Whitey would meet with Connolly and "be a good listener." It wasn't long before Whitey Bulger was giving Connolly money and buying gifts including unset diamonds for Connolly to give to his fiancee as an engagement present, Martorano said.

Prosecutors and law enforcement officials say it was the corrupt relationship between Whitey Bulger and Connolly that allowed Bulger's criminal enterprise to grow and even facilitated some murders.

The trial, which enters its fifth day Tuesday, is expected to take up to three months and has the potential to reveal sensational details about the mob and FBI corruption, especially if Bulger chooses to testify.

Bulger was in hiding for 16 years before he was captured in Santa Monica, California, two years ago, living under a false name with his girlfriend in an apartment.

At his July 2011 arraignment, he pleaded not guilty to the 19 murder charges and 13 other counts.