DOVER, N.H. — A defense expert in the case of a man charged with killing a 19-year-old University of New Hampshire student on Monday disputed the key witness’s testimony about how she died.

Dr. Ira Kanfer was the second defense witness to testify for 31-year-old Seth Mazzaglia, who is charged with first-degree murder in the October 2012 death of Elizabeth “Lizzi” Marriott.

Kanfer says that if Mazzaglia strangled Marriott from behind with a rope, there would have been a visible furrow on her neck where Mazzaglia’s former girlfriend, Kathryn McDonough, testified she saw no deep line.

Kanfer said descriptions of the neck area by McDonough and a couple Mazzaglia called to his apartment while Marriott’s body was still there are more consistent with a smothering death.

Six days after the killing, McDonough told Mazzaglia’s defense team that Marriott died of suffocation during rough, consensual sex with her. She shifted the blame to Mazzaglia six months later in her grand jury testimony, after she was granted immunity from prosecution for any role she played in the killing. She testified that Mazzaglia killed Marriott and raped her motionless body after Marriott had rebuffed two sexual advances.

The former defense investigator, Lisa Greenwaldt, testified Monday that she wished she had recorded her Oct. 15, 2012, interview with McDonough. Instead, Greenwaldt took notes during the nearly four-hour interview and used them to write a 25-page report that McDonough reviewed two days later.

Greenwaldt, the defense’s first witness in the 18-day trial, videotaped McDonough reviewing and editing the report. Defense attorney Joachim Barth asked her why she did that.

“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.”

Mazzaglia’s fate hinges largely on whether the jury finds McDonough’s witness account credible.

Kanfer testified during cross-examination that he is paid $4,000 a day to testify.

He also discussed his involvement in another controversial New Hampshire case — the 2003 death of Patric McCarthy in Lincoln. The New Hampshire medical examiner determined that McCarthy died of hypothermia after getting lost in the woods. His family maintains he was killed and hired Kanfer and other experts to review the autopsy report. Kanfer said the boy was smothered.

In 2011, the U.S. attorney and the attorney general closed out their investigations, concluding McCarthy died of hypothermia.

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 is serving a 1½- to 3-year sentence for hindering the prosecution, conspiracy and witness tampering.

Testimony resumes Tuesday.