DOVER, N.H. — Defense lawyers and prosecutors stressed one thing throughout the trial of a 31-year-old man convicted of raping and killing a University of New Hampshire student: The key was the truthfulness of the defendant’s ex-girlfriend.

The jury Friday decided that Kathryn McDonough, the star witness in Seth Mazzaglia’s murder trial, was telling the truth, convicting Mazzaglia of first- and second-degree murder in the death of 19-year-old Elizabeth “Lizzi” Marriott in October 2012.

McDonough, 20, first told investigators that she lured Marriott to their apartment as a sexual offering for Mazzaglia and she died while the women were engaged in consensual rough sex. She later testified that she made the story up because she still loved Mazzaglia and wanted to protect him. After getting immunity from prosecution, she testified that Mazzaglia choked then raped Marriott after she twice rejected his sexual advances. Marriott’s body has never been found.

McDonough has already pleaded guilty to hindering prosecution and is serving 1 ½ to 3 years in prison. The plea deal was contingent on her telling the truth at trial.

Defense lawyers painted her as an opportunistic liar who killed Marriott then changed her story when she found out she could cut a deal and get less time in prison if she pinned the crime on her ex-boyfriend.

“The main thoughts were that she was a pathological liar most of her life and up until she took that witness stand, her life had been based upon lie after lie after lie,” juror Maria Clifford, 52, said of McDonough. “I think Mr. Mazzaglia had control over her and she was afraid of him. We just thought, ‘She’s on the witness stand now and she doesn’t have anything left to lose,’ and she came to the realization that ‘I need to do the right thing.'”

Clifford also said McDonough’s efforts to reconnect with her mother after being estranged from her influenced jurors.

The verdict was bittersweet for Marriott’s father, Bob, who read a statement in a shaky voice that broke several times. Mazzaglia’s life sentence without chance of parole will never soothe their grief, he said.

“We will always miss her and we wonder what could have been,” Marriott said. “In fact, the trial has been torturous for us. The truth of what happened to Lizzi is horrendous. And every time it’s been told, it has reinforced the despair that we feel.”

Marriott said the verdict, which included convictions on lesser charges of conspiracy, will keep a dangerous man off the streets and protect other women.

He also had harsh words for Mazzaglia’s lawyers for what he called intentionally misstating his daughter’s actions the night she died.

“Blaming a victim who is unable to defend herself is a typical ploy used by defense teams. If you are dead, you cannot correct a mischaracterization,” he said.

Mazzaglia showed no emotion as the verdict from the jury of seven women and five men was read. He was led from the courtroom in handcuffs. His lawyers did not comment after the verdict.

Clifford, the juror, said she approached the trial as if Mazzaglia was one of her own family members.

“Everybody deserves a fair trial, no matter what they did,” she said.

Clifford said the jury looked at all the evidence and “just felt that the prosecution just made a really strong case.” She hopes the verdict helps the Marriott family, even a little.

“They’ll never forget what happened but maybe they’ll get a little more peace knowing the murderer is behind bars,” she said.