You can now read 10 articles in a month for free on BostonGlobe.com. Read as much as you want anywhere and anytime for just 99¢.

The Boston Globe

Metro

Defense in N.H. murder trial focuses on ex-girlfriend

Lisa Greenwaldt, an investigator with the N.H. Public Defenders Office, testified during the trial of Seth Mazzaglia on Monday.

Rich Beauchesne/Portsmouth Herald/AP

Lisa Greenwaldt, an investigator with the N.H. Public Defenders Office, testified during the trial of Seth Mazzaglia on Monday.

DOVER, N.H. (AP) — A private investigator hired by a man charged with raping and killing a University of New Hampshire student told a jury Monday she wished she had recorded an interview in which the main prosecution witness — the defendant’s ex-girlfriend — said she had killed the woman accidentally.

Six days after the killing, Lisa Greenwaldt interviewed the former girlfriend of 31-year-old Seth Mazzaglia, who is charged with first-degree murder in the October 2012 death of 19-year-old Elizabeth ‘‘Lizzi’’ Marriott of Westborough, Massachusetts.

Seth Mazzaglia (right) and his attorney, Melissa Davis,  at the trial on Monday.

Rich Beauchesne/Portsmouth Herald/AP

Seth Mazzaglia (right) and his attorney, Melissa Davis, at the trial on Monday.

Continue reading below

Greenwaldt said 20-year-old Kathryn McDonough told her that she suffocated Marriott during consensual rough sex. After being granted immunity from prosecution 18 months later, McDonough told a grand jury that Mazzaglia strangled Marriott with a rope then raped her motionless body after Marriott twice rebuffed his sexual advances.

Greenwaldt is the defense team’s first witness on day 18 of a trial that has been rife with sordid testimony about bondage, sexual dominance and sadomasochism.

Prosecutor Peter Hinckley in his cross-examination punctuated most of his questions with references to the ‘‘unrecorded interview.’’ Greenwaldt agreed that jurors would not have the benefit of hearing her questions and what information they may have conveyed to McDonough. Prosecutors say McDonough concocted her original story to try to protect the man she still loved.

Jurors saw a videotape Greenwaldt made of McDonough reviewing a summary of what she told the defense team, but not the original 3 1/2-hour interview.

‘‘I wish I had recorded her first interview. I really do,’’ Greenwaldt testified.

Continue reading below

She said the video she made of McDonough reading and editing Greenwaldt’s 25-page summary of her statement marked the first time she had ever done that. Defense attorney Joachim Barth asked Greenwaldt why she did it.

‘‘Given what she told us and her demeanor telling us, we wanted to memorialize it,’’ Greenwaldt said. ‘‘Many times people who incriminate themselves, they change their story.’’

McDonough testified for 10 days. She told jurors she lured Marriott to the Dover apartment she shared with Mazzaglia because he wanted other women in their sex life. She said the three of them played a game of strip poker, and then Mazzaglia suggested Marriott and McDonough kiss. She testified Marriott rejected that notion and another that Marriott watch while he had sex with McDonough.

It was while she and Marriott were watching a movie, McDonough testified, that Mazzaglia came up behind them, slipped a rope around Marriott’s neck and pulled it until she lost consciousness. The pair then used Marriott’s car to transport her body to Peirce Island in Portsmouth and throw it into a river that feeds into the ocean. It has not been found.

McDonough’s serving a 1-1/2 to 3-year sentence for hindering the prosecution, conspiracy and witness tampering.

You have reached the limit of 10 free articles in a month

Stay informed with unlimited access to Boston’s trusted news source.

  • High-quality journalism from the region’s largest newsroom
  • Convenient access across all of your devices
  • Today’s Headlines daily newsletter
  • Subscriber-only access to exclusive offers, events, contests, eBooks, and more
  • Less than 25¢ a week