Stephen “The Rifleman” Flemmi testified Friday that his former partner James “Whitey” Bulger killed Flemmi’s girlfriend Debra Davis in 1981 because she had learned about their corrupt relationship with the FBI, and Bulger feared she could jeopardize it.

“It’s going to affect me until the day I die,” the 79-year-old Flemmi said.

He said he opposed killing the 26-year-old Davis, who he said he loved. But he accepted the plan to kill her after Bulger outlined that because she knew about their relationship with the FBI, that she or her brothers could jeopardize it if they told anyone.

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Flemmi said he brought Davis to a house he recently bought his mother in South Boston. When inside, Bulger came from a side room and began strangling her. He dragged her to the basement, where she ultimately died. They later buried her remains in a secret grave by the Neponset River in Quincy.

Flemmi had testified throughout the day about crimes he committed with Bulger, including other murders, but the description of the death of Davis sent a chill through the courtroom. The 83-year-old Bulger look red-faced but did not appear to look at his old partner. Bulger has adamantly denied killing anyone or that he was an informant.

Outside the courthouse, Davis’s brother, Steve said, “It brings tears to my eyes but I learn how to deal with it, fight it, because I need to learn more about it.”

Flemmi testified earlier that Bulger was involved in several murders, including the 1975 killing of bar owner Edward Connors in a phone booth on Morrissey Boulevard.

Flemmi said he and Bulger shot Connors, who they believed was talking too much. “We just stepped up from aside the building, the garage, and shot him,” said Flemmi.

Bulger emptied both barrels of a shotgun, then pulled out a pistol and kept firing at Connors, Flemmi said. Flemmi said he fired about six shots.

Flemmi said Bulger told him he also killed Donald McGonagle, mistaking him for his brother Paul, member of a rival gang, in 1969. The Bulger gang also killed several other men, Flemmi testified.

Flemmi testified in lurid detail about his lengthy criminal career, including other murders he committed without Bulger; their involvement with corrupt FBI agents; and their gang’s structure and operations.

He described their criminal activities as bookmaking, loansharking, extortion.

“Murder,” Assistant US Attorney Fred Wyshak asked.

“Oh, definitely,” Flemmi responded.

In one case, he said, the late FBI agent H. Paul Rico gave him information so he and others could kill George McLaughlin, member of a rival gang.

In another case, Flemmi told jurors, he and Bulger, who had formed a partnership with John J. Connolly Jr., a corrupt FBI agent, were informed by Connolly that Revere nightclub owner Richard Castucci was informing on them.

“Collectively, we made a decision … kill Richard Castucci,” Flemmi said. He said the decision was made by the leaders of Bulger’s Winter Hill Gang, and hit man John Martorano was the one who killed Castucci. He helped dispose of the body.

“We all decided, because we were a group, we all decided that if anything were to be accomplished, we had to agree to it,” Flemmi said.

Flemmi said Bulger approached the gang about meeting with Connolly after the agent – a friend of his brother William, a respected politician from South Boston at the time – reached out to him.

Bulger told associates that Connolly told him that “the Mafia has their sources and we can help you. If they want to play checkers, we can play chess,” according to Flemmi.

On Thursday, in their first encounter in 18 years, Flemmi greeted Bulger with an expletive after taking the stand in US District Court and describing his former partner as an FBI informant who gave information to law enforcement “hundreds of times’’ during their shared criminal careers.

Flemmi is a key witness in Bulger’s racketeering trial, and on Thursday prosecutors got straight to the crux of his testimony: his partnership with Bulger and their relationship with the FBI.

“Was Mr. Bulger an informant?” Assistant US Attorney Fred Wyshak asked.

Flemmi said he was.

“What was the nature of your relationship?” Wyshak asked.

“Strictly criminal,” Flemmi responded.

Flemmi was called to testify at the end of the trial day and was on the stand for less than 15 minutes. But before he left and after the jury had left the courtroom, he stood in the witness stand with his hands on his hips and hissed “mother [expletive]” at Bulger.

The 83-year-old Bulger responded, although it was unclear what he said.

The two men, who allegedly mapped out extortion and murders together, continued to glare at each other.

“Really?” Flemmi said, before being led from the courtroom.

Flemmi is perhaps the most anticipated witness in the trial, the man who was by Bulger’s side as they allegedly committed murder and mayhem for decades.

Bulger is charged in a sweeping federal racketeering indictment with participating in 19 murders, extortion, money laundering, and stockpiling guns in the 1970s and 1980s. Prosecutors say he got away with his crimes for so long because he was protected by corrupt FBI handlers. Bulger has said that the allegations he most wants to refute are that he was an informant and that he killed two women.

In addition to his testimony that Bulger was an informant, Flemmi is expected to testify about the killing of the two women.

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