NEW BRUNSWICK, N.J. — For 20 minutes, Melanie McGuire tried to convince a suspicious friend that she had nothing to do with her husband's murder. She used a matter-of-fact tone, then sarcasm, profanity, anger and tears, but the friend, a nursing school classmate, was unrelenting. You must know something, he told her.

Finally, McGuire laid down her highest card. She swore her innocence on her two young sons.

"On my kids. I am telling you, on my kids," she said with a sigh. "Do you believe me now?"

For a moment, there was silence and then her friend spoke.

"No, I don't," he said.

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The exchange was part of a tense phone conversation secretly recorded by the friend, James Finn, and played by prosecutors Tuesday at the first-degree murder trial of McGuire, a 34-year-old fertility clinic nurse. (VIDEO)

Finn, a registered nurse at a blood bank who had carried a torch for McGuire since their university days, told jurors that he agreed to try to elicit incriminating evidence during the phone calls after detectives investigating her husband's 2004 murder and dismemberment convinced him she was guilty.

He testified that the investigators stood by his side during the half-hour call in April 2005, silently feeding him questions and suggesting approaches.

During the call, Finn told McGuire he had been summoned to an interview with detectives and was terrified about the meeting which he called "going into the lion's den." He repeatedly asked her to tell him the truth about William McGuire's death and specifically pressed her for the whereabouts of a .38-caliber gun that she had purchased before the murder.

She denied hiding anything from him. She said she did not know where the gun was and urged him to tell police the truth, saying that nothing he knew was going to get him or her in trouble.

But Finn said he was convinced she was holding something back.

"Like what? That I killed my husband?" she asked with a note of incredulity.

"Did you?" he asked.

"No. Thanks for asking," she snapped.

Finn persisted, telling McGuire, who had rejected several declarations of his love in nursing school, that he was only seeking information to help her.

"Melanie, I love you. I'll do anything for you. Tell me," he pleaded.

"I didn't f---ing do it," she answered.

When he later reiterated his disbelief, McGuire broke into sobs.

Jurors appeared riveted by the recording and the rest of Finn's testimony. The panelists followed a transcript of the call intently with a few jurors opting for headphones to enhance the audio quality.

Finn described how his school friendship with McGuire in the mid-1990s quickly turned to infatuation.

"I eventually thought she was the best thing since sliced bread, fell madly in love with her,  actually," he said.

McGuire gazed toward him from the defense table as he testified. Occasionally, she whispered to one of her lawyers or scribbled a note. The pair only seemed to make eye contact once, when Finn first took the stand.