BOSTON (WHDH) — Accused Boston mob boss James “Whitey” Bulger hasn’t laid eyes on his old partner, Stephen “The Rifleman” Flemmi, since he skipped town in 1994 on the cusp of being indicted.

James "Whitey" Bulger, who has pleaded not guilty, is accused of participating in 19 murders and a range of other crimes involving money-laundering, extortion, drugs and weapons. If he's convicted of just two counts, he could go to prison for life.

That will change this week — and if the recent past is any indication, Flemmi’s turn as star prosecution witness at Bulger’s racketeering and murder trial could be explosive.

When Bulger’s former protégé, Kevin Weeks, took the stand against him last week, hard stares were exchanged and expletives flew across the federal courtroom.

Bulger, 83, also lost his cool when retired FBI agent John Morris testified the gangster had secretly worked as an informant, calling him “a [expletive] liar,” according to prosecutors.

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Flemmi, 78, was the No. 2 man in the Winter Hill Gang but he stayed put in Boston to face federal charges while Bulger went on the lam for 16 years.

When he pleaded guilty in 2003, admitting to 10 murders, the brother of one of his victims became so enraged he started yelling and was escorted from the courtroom.

“It was hard to take,” said Steve Davis, brother of Flemmi's girlfriend Debra Davis, who was allegedly strangled in 1981 and buried on a riverbank because she knew too much.

Davis said he would try to control himself when Flemmi makes his return to Boston – but no promises.

“I’m disgusted thinking about him,” he said. “Guy makes me sick.”

And, he said, no one knows how Bulger will react to having his former right-hand-man working to put him behind bars for the rest of his life.

"This could be the boiling point of the whole trial," said Davis, who has not missed a day since the trial began with opening statements June 12.

While Bulger was living under an assumed name in California, Flemmi testified in 2009 at a civil trial on lawsuits filed by victims against the FBI and said Whitey wanted Davis dead out of jealousy and fear.

"Bulger kind of resented the fact that I didn't spend enough time with him," Flemmi told the court. "He would contact me and I wouldn't respond."

He said that when Bulger learned that Flemmi had told Davis about their relationship with the FBI, she was doomed.

"He wanted to kill her," Flemmi said.

For this trial, Flemmi will follow a rogue’s gallery of state witnesses to the stand, including hitman John “The Executioner” Martorano, who served just a dozen years in prison after admitting to 20 murders.

Courtesy Steve Davis

Debra Davis, ex-girlfriend and murder victim of Stephen Flemmi.

Like Martorano, Flemmi is expected to detail a number of bloody crimes he claims he committed with Bulger, the slaying of Debra Davis among them.

What will really anger Bulger, however, is if Flemmi joins other witnesses in portraying his old friend as informant who snitched for the FBI.

When Weeks called him a "rat," Bulger cursed at him in the courtroom.

Davis' hope is that whatever Flemmi says on the stand will infuriate Bulger so much that he feels like he needs to testify and tell his side of the story — the only way he thinks he can get answers about his sister's death.

If not, he'll have to settle for Bulger stewing at the defense table while Flemmi relives the past. Does Davis think that Bulger will hurl a few choice words at Flemmi like he did with Weeks?

"I don’t know," he said. "But that was fun."

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