BOSTON — A man who was allegedly killed by James "Whitey" Bulger predicted he would be killed if Bulger learned he'd been cooperating with authorities, a former FBI agent testified Monday.

Retired agent Gerald Montanari told jurors in Bulger's racketeering case that Edward "Brian" Halloran became an FBI informant in 1982 and agreed to testify against Bulger and members of his gang in the 1981 slaying of Tulsa, Okla., businessman Roger Wheeler, as well as other killings.

Halloran was facing a state murder charge and hoped his cooperation would help him in that case – and secure protection for himself and his family.

Halloran told the former agent Bulger or his partner, Stephen "The Rifleman" Flemmi, would probably kill him if they found out he was cooperating, Montanari said.

"He said that if Bulger or Flemmi had any indication that he was cooperating with the FBI that they would go to any extreme, even if it meant killing innocent bystanders, including his family," Montanari said.

Bulger is charged with opening fire on a car Halloran was in a few months after he turned informant. Halloran and Michael Donahue, an acquaintance who had offered him a ride home, were killed.

Bulger, 83, has pleaded not guilty to participating in 19 killings during the 1970s and `80s while he allegedly led the Winter Hill Gang. He has also denied prosecutors' assertions that he was a longtime FBI informant who ratted on the rival New England Mafia and other criminals.

Montanari said Halloran was a "mid-level strong-arm type" who acted as an enforcer for the gang. He said Halloran reached out to the FBI at the end of 1981 about Wheeler's death.

The FBI made arrangements for Halloran and his family to live in a rented house on Cape Cod to protect him, Montanari said. He said the FBI believed if Halloran abided by the guidelines given to him, including that he not go to Boston, "we felt this would be sufficient security for him."

Montanari said he and another agent met with Halloran dozens of times over a period of two to three months, and he told them about various crimes he said were committed by Bulger, Flemmi, hit man John Martorano and others. Halloran had agreed to wear a recording device to capture conversations he had with a Bulger associate who played a role in the Wheeler killing, Montanari said.

Prosecutors say Bugler learned of Halloran's cooperation through former FBI agent John Connolly, who was Bulger's handler while he was an FBI informant. Halloran was gunned down in May 1982, near a restaurant in South Boston.

Montanari said the FBI told Halloran before his death that they were closing him as an informant because he would not take a lie detector test and was waffling on whether to go into the witness protection program.

In other testimony Monday, Martorano's former girlfriend, Patricia Carlson, described living with Martorano while he was on the run after he was indicted in the late 1970s.

Carlson, who was then Patricia Lytle, said she began dating Martorano when she was 15 and he was 35. She said one day he asked her if she wanted to go to Florida for a vacation. "We ended up staying there 20 years," she said.

They had a son together, James Stephen, now 27. Martorano testified earlier that his son was named after Bulger and Flemmi.

Carlson described a series of cash payments she received from Bulger's gang, including a $10,000 payment that was left at her mother's house in July 1996 after Martorano was arrested.

She also testified that she told numerous lies when she appeared before a federal grand jury in 1995.

"I was worried that I was going to cause more problems for Johnny. I was concerned with what was going to happen with my 8-year-old boy, and Johnny told me to lie," she said.

Carlson said she only recently admitted she lied after she spoke to her attorney and he told her that the statute of limitations for perjury had expired.

Under cross-examination by Bulger attorney Hank Brennan, Carlson said that up until last month, the government had not contacted her since her 1995 grand jury testimony to provide information about Martorano's illegal activities.

In other testimony, Dr. Richard Evans, the retired chief medical examiner for Massachusetts, reviewed reports in all 19 killings and listed the cause of death for each one. He said most of the victims had multiple gunshot wounds.

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  • Whitey Bulger, Kevin Weeks

    This undated surveillance photo released Monday, July 8, 2013, by the U.S. Attorney's office at federal court in Boston shows James "Whitey" Bulger, left, walking with his former right hand man, Kevin Weeks. Weeks took the witness stand Monday at Bulger's racketeering trial and described a double slaying, multiple extortions and drug dealing. (AP Photo/U.S. Attorney's Office)

  • Whitey Bulger, Kevin Weeks

    This undated surveillance photo released Monday, July 8, 2013, by the U.S. Attorney's office at federal court in Boston shows James "Whitey" Bulger, left, with his former right hand man, Kevin Weeks. Weeks took the witness stand Monday at Bulger's racketeering trial and described a double slaying, multiple extortions and drug dealing. (AP Photo/U.S. Attorney's Office)

  • Stephen Rakes

    File - In this Tuesday, June 12, 2012, file photo Stephen Rakes receives a handshake outside federal court in Boston. Authorities say Rakes, who was on the witness list for the racketeering trial of reputed mobster James "Whitey" Bulger, has died. The Middlesex District Attorney’s Office says Stephen Rakes was found dead Wednesday, July 17, 2013, about 1:30 p.m. in Lincoln, Mass., with no obvious signs of trauma to his body. (AP Photo/Elise Amendola, File)

  • Tommy Donahue, Patricia Donahue

    Tommy Donahue, son of alleged murder victim Michael Donahue, walks with his mother, Patricia Donahue, as they leave U.S. District Court in Boston Friday, July 19, 2013 after the day's session ended in the murder and racketeering trial of James "Whitey" Bulger. (AP Photo/Elise Amendola)

  • Stephen Rakes

    Pallbearers remove the casket of Stephen "Stippo" Rakes from a funeral home in Boston's South Boston neighborhood, Thursday, July 25, 2013. Authorities said a jogger discovered the body of the 59-year-old Rakes Wednesday, July 17, 2013 in the woods along a street in Lincoln, Mass. Reputed mobster James "Whitey" Bulger is accused of forcing Rakes and his former wife to sell their store in 1984 to use as a headquarters for his gang and as a source of legitimate income. (AP Photo/Steven Senne)

  • Whitey Bulger

    This undated black and white photo filed in federal court documents in Boston by defense attorneys for James "Whitey" Bulger on Wednesday, July 31, 2013, shows Bulger in an unknown location. The photo was among several that showed a softer side of Bulger, which prosecutors complained were an attempt to salvage his reputation. Bulger, 83, is charged in a racketeering indictment with playing a role in 19 killings and multiple extortions during the 1970s and ‘80s when he alleged led the Winter Hill Gang. (AP Photo/Federal Court Documents)

  • Whitey Bulger

    This undated photo filed in federal court documents in Boston by defense attorneys for James "Whitey" Bulger on Wednesday, July 31, 2013, shows Bulger with holding a goat in an unknown location. The photo was among several that showed a softer side of Bulger, which prosecutors complained were an attempt to salvage his reputation. Bulger, 83, is charged in a racketeering indictment with playing a role in 19 killings and multiple extortions during the 1970s and ‘80s when he alleged led the Winter Hill Gang. (AP Photo/Federal Court Documents)

  • Whitey Bulger

    This undated photo filed in federal court documents in Boston by defense attorneys for James "Whitey" Bulger on Wednesday, July 31, 2013, shows Bulger in an unknown location. The photo was among several that showed a softer side of Bulger, which prosecutors complained were an attempt to salvage his reputation. Bulger, 83, is charged in a racketeering indictment with playing a role in 19 killings and multiple extortions during the 1970s and ‘80s when he alleged led the Winter Hill Gang. (AP Photo/Federal Court Documents)

  • Whitey Bulger

    This undated photo filed in federal court documents in Boston by defense attorneys for James "Whitey" Bulger on Wednesday, July 31, 2013, shows Bulger with an unidentified woman holding birds in an unknown location. The photo was among several that showed a softer side of Bulger, which prosecutors complained were an attempt to salvage his reputation. Bulger, 83, is charged in a racketeering indictment with playing a role in 19 killings and multiple extortions during the 1970s and ‘80s when he alleged led the Winter Hill Gang. (AP Photo/Federal Court Documents)

  • Tommy Donahue

    Tommy Donahue, son of alleged murder victim Michael Donahue, speaks to news media outside U.S. District Court in Boston Monday, August 5, 2013. The jury is expected to begin deliberating Tuesday in the racketeering case against 83-year-old Whitey Bulger, whose 16 years on the run embarrassed the FBI and exposed the bureau's corrupt relationship with its underworld informants. (AP Photo/Elise Amendola)

  • The motorcade carrying defendant James "Whitey" Bulger leaves federal court in Boston Monday, Aug. 5, 2013. Bulger's lawyers used their closing arguments Monday to go after three gangsters who took the stand against the reputed Boston crime boss, portraying them as pathological liars whose testimony was bought and paid for by prosecutors. (AP Photo/Elise Amendola)

  • James "Whitey" Bulger

    In this courtroom sketch, James "Whitey" Bulger listens to defense attorney, Hank Brennan, during closing arguments at U.S. District Court, in Boston, Monday, Aug. 5, 2013. Bulger's lawyers used their closing arguments to go after three gangsters who took the stand against the reputed Boston crime boss, portraying them as pathological liars whose testimony was bought and paid for by prosecutors. (AP Photo/Jane Flavell Collins)

  • Fred Wyshak, James "Whitey" Bulger

    In this courtroom sketch, prosecutor Fred Wyshak, standing, speaks during closing arguments in the trial of James "Whitey" Bulger, right, at U.S. District Court, in Boston, Monday, Aug. 5, 2013. A federal prosecutor summed up the government's case by calling Bulger "one of the most vicious, violent and calculating criminals ever to walk the streets of Boston," and urged the jury to convict him of charges that include 19 killings committed during the 1970s and '80s. (AP Photo/Jane Flavell Collins)

  • Hank Brennan, James "Whitey" Bulger

    In this courtroom sketch, James "Whitey" Bulger, right, listens to his attorney, Hank Brennan, during closing arguments at U.S. District Court, in Boston, Monday, Aug. 5, 2013. Bulger's lawyers used their closing arguments to go after three gangsters who took the stand against the reputed Boston crime boss, portraying them as pathological liars whose testimony was bought and paid for by prosecutors. (AP Photo/Jane Flavell Collins)

  • J.W. Carney Jr., James "Whitey" Bulger

    In this courtroom sketch, James "Whitey" Bulger, right, listens to his defensive attorney, J.W. Carney Jr., during closing arguments at U.S. District Court, in Boston, Monday, Aug. 5, 2013. Bulger's lawyers used their closing arguments to go after three gangsters who took the stand against the reputed Boston crime boss, portraying them as pathological liars whose testimony was bought and paid for by prosecutors. (AP Photo/Jane Flavell Collins)

  • FILE - In this June 6, 2013 photo, Stephen Rakes smiles outside the liquor store he once owned in the South Boston neighborhood of Boston. Rakes, who said he had been extorted by James "Whitey" Bulger and hoped to testify at his trial, was given iced coffee laced with cyanide that killed him, authorities said Friday, Aug. 2, 2013, and that 69-year-old William Camuti, of Sudbury, is charged with attempted murder in his death. Rakes' body was found July 17 in the woods in suburban Boston the day after he learned he would not be called to testify. (AP Photo/Michael Dwyer, File)

  • James "Whitey" Bulger

    FILE - This June 23, 2011 file booking photo provided by the U.S. Marshals Service shows James "Whitey" Bulger. A new judge has been named to preside over the trial of reputed Boston gangster James "Whitey" Bulger a day after the previous judge was removed to eliminate any appearance of bias. The clerk of the federal court Friday, March 15, 2013 announced the appointment of U.S. District Judge Denise J. Casper. (AP Photo/U.S. Marshals Service, File)

  • FILE - This is a 1986 FBI handout file photo of New England organized crime figure James J. "Whitey" Bulger. Bulger, a notorious Boston gangster on the FBI's "Ten Most Wanted" list for his alleged role in 19 murders, has been captured near Los Angeles after living on the run for 16 years, authorities said Wednesday, June 22, 2011. (AP Photo/FBI, File)

  • FILE - In these 1984 file photos originally released by the FBI, New England organized crime figure James "Whitey" Bulger is shown. Bulger, a notorious Boston gangster on the FBI's "Ten Most Wanted" list for his alleged role in 19 murders, has been captured near Los Angeles after living on the run for 16 years, authorities said Wednesday June 22, 2011. (AP Photo/Federal Bureau of Investigation, File)

  • FILE - The photos left and center are FBI handout photos of fugitive Massachusetts mobster James "Whitey" Bulger, taken in the 1980s. At right is an FBI photo of Bulger's girlfriend, Catherine Greig, who travels with the former gangster. Bulger, a notorious Boston gangster on the FBI's "Ten Most Wanted" list for his alleged role in 19 murders, has been captured near Los Angeles after living on the run for 16 years, authorities said Wednesday June 22, 2011. (AP Photo/FBI Handouts, File)

  • FILE - In this 1995 file photo provided by the FBI, fugitive mobster James "Whitey" Bulger is shown in a photo released Saturday, April 17, 2004, and taken shortly before he disappeared in 1995. Bulger, a notorious Boston gangster on the FBI's "Ten Most Wanted" list for his alleged role in 19 murders, has been captured near Los Angeles after living on the run for 16 years, authorities said Wednesday June 22, 2011. (AP Photo/Federal Bureau of Investigation, File)

  • FILE - This is an undated handout file photo the FBI released in this Dec. 30, 1998 showing reputed Boston mobster and fugitive James J. "Whitey" Bulger. Bulger, a notorious Boston gangster on the FBI's "Ten Most Wanted" list for his alleged role in 19 murders, has been captured near Los Angeles after living on the run for 16 years, authorities said Wednesday June 22, 2011. (AP Photo/FBI Handout, File)

  • Catherine Greig

    FILE - This undated file photo provided by the U.S. Marshals Service shows Catherine Greig, the longtime girlfriend of Whitey Bulger, who was captured with Bulger June 22, 2011, in Santa Monica, Calif. Greig faces a maximum of 15 years in prison when she is sentenced Tuesday, June 12, 2012 for helping to hide Bulger for 16 years. (AP Photo/U.S. Marshals Service, File)

  • Anna Bjornsdottir

    Bjornsdottir, a former Miss Iceland, was 57 when she collected a $2 million reward for giving the FBI a tip that led to the arrest of Bulger and Greig in 2011. Bjornsdottir recognized the fugitive couple as neighborhood residents she knew in Santa Monica.

  • BULGER

    FILE - In this March 10, 2003 file photo, University of Massachusetts President William Bulger speaks before a joint legislative Ways and Means Committee hearing at Bridgewater State College, Monday, March 10, 2003 in Bridgewater, Mass. Bulger called the governor's proposal to reorganize the state's public college and university system an "attack on public higher education." (AP Photo/Lisa Poole, file)

  • This 2011 photo provided by the U.S. Attorney's office shows identification cards found in the Santa Monica, Calif., apartment where Whitey Bulger and Catherine Greig hid before their arrest in June 2011. The photo was among hundreds of documents unsealed by prosecutors Friday, June 15, 2012, three days after Greig was sentenced in Boston to eight years in prison for helping Bulger during his years as a fugitive. (AP Photo/U.S. Attorney's Office)

  • This 2011 photo provided by the U.S. Attorney's office shows a shelf containing books about gangsters and crime in the Santa Monica, Calif., apartment where Whitey Bulger and Catherine Greig hid before their arrest in June 2011. The photo was among hundreds of documents unsealed by prosecutors Friday, June 15, 2012, three days after Greig was sentenced in Boston to eight years in prison for helping Bulger during his years as a fugitive. (AP Photo/U.S. Attorney's Office)

  • This 2011 photo provided by the U.S. Attorney's office shows guns displayed in the Santa Monica, Calif., apartment where Whitey Bulger and Catherine Greig hid before their arrest in June 2011. The photo was among hundreds of documents unsealed by prosecutors Friday, June 15, 2012, three days after Greig was sentenced in Boston to eight years in prison for helping Bulger during his years as a fugitive. (AP Photo/U.S. Attorney's Office)

  • This 2011 photo provided by the U.S. Attorney's office shows a 1999-2001 New York driver's license with a photo of Catherine Greig, found in the Santa Monica, Calif., apartment where Greig and Whitey Bulger hid before their arrest in June 2011. The photo was among hundreds of documents unsealed by prosecutors Friday, June 15, 2012, three days after Greig was sentenced in Boston to eight years in prison for helping Bulger during his years as a fugitive. (AP Photo/U.S. Attorney's Office)

  • J.W. Carney Jr

    FILE - In this July 6, 2011 file photo, defense attorney J.W. Carney Jr., appointed to represent James "Whitey" Bulger, speaks to the media as he leaves federal court after Bulger's arraignment on 19 counts of murder in Boston. Carney on Tuesday, Jan. 8, 2013 framed an attempt to get the presiding judge kicked off his client's murder case as an effort to protect the court's integrity. (AP Photo/Stephan Savoia, File)

  • Steven Davis, the brother of Debra Davis, a woman allegedly killed by James "Whitey" Bulger, stands near the Neponset River where his sister's body was found in Quincy,Mass., Thursday, June 21, 2012. Davis would like to construct a memorial for his sister and others killed by the Boston mobster adjacent to the river. (AP Photo/Stephan Savoia)

  • Carmen Ortiz, Richard DesLauriers

    U.S. Attorney Carmen Ortiz speaks to reporters as FBI Special Agent Richard DesLauriers, rear left, watches outside federal court in Boston on Tuesday, June 12, 2012. Catherine Greig, who spent 16 years on the run with former Boston mobster James "Whitey" Bulger, was sentenced to eight years in prison for helping to hide one of the FBI's Ten Most Wanted Fugitives. (AP Photo/Elise Amendola)

  • Three sisters of Paul McGonagle leave a plea hearing for Catherine Greig, the longtime girlfriend of reputed Boston mob boss James "Whitey" Bulger, outside the Federal Courthouse in Boston, Wednesday afternoon, March 14, 2012. Paul McGonagle is alleged to have been one of the 19 murders Bulger is accused of committing. (AP Photo/Stephan Savoia)

  • Steve Davis, Tommy Donahue

    Victims' family members, Steve Davis, left, and Tommy Donahue react outside federal court in Boston Tuesday, June 12, 2012, after Catherine Greig, who spent 16 years on the run with former Boston mobster James "Whitey" Bulger, was sentenced to eight years in prison for helping to hide one of the FBI's Ten Most Wanted Fugitives. (AP Photo/Elise Amendola)

  • Howard "Howie" Winter, left and James Melvin talk after their arraignment at Somerville District Court on Friday, June 8, 2012 in Somerville, Mass. Winter, 83, and Melvin, 70, were arrested Thursday after authorities said they tried over several months to extort $35,000 from each of two men who had arranged a $100,000 loan for a third man. Both men pleaded not guilty to attempted extortion and conspiracy charges. Winter is the former head of the Winter Hill Gang, that was later run by James "Whitey" Bulger. (AP Photo/The Boston Globe, Jonathan Wiggs, Pool)

  • In this book cover image released by Forge Books, "Betrayal: Whitey Bulger and the FBI Agent who Fought to Bring him Down," by Robert Fitzpatrick with Jon Land, is shown. (AP Photo/Forge Books)

  • Stephen Flemmi

    FILE - In this Sept. 22, 2008, file photo, Stephen "The Rifleman" Flemmi, a jailed Boston mob leader, testifies in a Miami court room in the murder trial of former FBI agent John Connolly. Flemmi is on the witness list to testify at the trial of James "Whitey" Bulger, which begins with jury selection on Tuesday, June 4, 2013 in federal court in Boston. (AP Photo/J. Pat Carter, File)

  • FILE-In this June 28, 2011 file photo, Kevin Weeks, a former top lieutenant to James "Whitey" Bulger, is shown during an interview with the Associated Press in Boston. Weeks is on the witness list to testify at Bulger's trial, which begins with jury selection on Tuesday, June 4, 2013 in federal court in Boston. (AP Photo/Stephan Savoia, File)

  • J.W. Carney Jr

    FILE - In this July 6, 2011 file photo, defense attorney J.W. Carney Jr., appointed to represent James "Whitey" Bulger, speaks to the media as he leaves federal court after Bulger's arraignment on 19 counts of murder in Boston. Carney on Tuesday, Jan. 8, 2013 framed an attempt to get the presiding judge kicked off his client's murder case as an effort to protect the court's integrity. (AP Photo/Stephan Savoia, File)