"I don't buy that euthanasia story. You killed your father," the man told Linda Roberts, who confessed to killing her father along with her sister, Mary Beth Tomaselli

Mary Beth Tomaselli, Linda Roberts
Sisters Mary Beth Tomaselli, left, and Linda Roberts
| Credit: Pinellas County Sheriff's Office (2)

The dead Anthony Tomaselli haunted the dreams of his daughter Linda Roberts.

"You don't know the pain that I go through over what happened with my father," she told a man who recently had broken off a relationship with her.

"She had dreams that her father was in the room and she was scared," the man tells PEOPLE.

"'Wait a minute, I don't get this,'" he told her. "'You tell me you loved your father so much, if he comes back to you in your room in a dream, why are you scared?' And she just couldn't answer."

Later those cryptic comments yielded a startling confession.

"I can't take it anymore. I'm going to tell you," Linda told the man, who asked that his name not be published. "She said, 'I killed my father.'"

"And then I took my phone out of my pocket," he says. "Because I knew I had to stop there and start recording."

But another shock was still to come. "Well, we did," Linda told him. "We did it together."

The other person? Linda's sister, Mary Beth Tomaselli.

The surreptitious recording, which the man who'd previously slept with both women shared the next day with Pinellas County Sheriff's detectives in Florida, launched an investigation that lead earlier this year to the sisters' pleading guilty to the second-degree murder of Anthony in March 2015.

(The case was featured on Thursday's night's episode of PEOPLE (the TV Show). An exclusive clip is shown below.)

RELATED VIDEO: 'Like Arsenic and Old Lace': Sisters in Their 60s Hid Secret of Dad's Murder — Until They Slept with Same Man

In what they tried to portray as a mercy killing of a sick, elderly man, both admitted to mixing what they believed would be a lethal dose of Linda's sleeping pills into their father's drink, according to court transcripts obtained by PEOPLE.

When that didn't work, Linda stuffed a washcloth in his mouth as Mary Beth held his nose shut to asphyxiate him, charging documents state.

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Before doing so, Linda gave Mary Beth's daughter Lauren Johnson, 41, who was staying with Anthony in his Palm Harbor condo, one of her sleeping pills so she wouldn't wake up and catch them, say detectives. The next morning the sisters then staged the scene to look like they had attempted CPR on Anthony before dialing 911.

Anthony Tomaselli
Anthony Tomaselli
| Credit: Pinellas County Sheriff's Office

Linda, 64, was sentenced to 20 years in prison. Mary Beth, 65, after offering to testify against her sister, was sentenced to 15 years.

Sisters Thought Father 'Was Going to Be a Burden to Them': Prosecutor

Before learning of the confession in February 2019, authorities said four years went by with no reason for them to suspect Anthony had been murdered. Linda and Mary Beth were never considered suspects. A medical examiner had concluded Anthony died of natural causes.

The sisters' purported discovery of their father's body brought an abrupt end to their push to move him — against his will — into assisted living. It came hours after Linda received a doctor's phone call reporting a mass on Anthony's lungs — which the sisters took to mean a likely return of his prior lung cancer.

"He had some normal health issues for somebody you might expect of that age, but he was in no way sick to the point that he was terminal," prosecutor Thomas Koskinas tells PEOPLE.

Still, he says, Linda and Mary Beth claimed "altruistic motives."

"They justified it, at least in their own mind, that they did it out of mercy to their father," says Koskinas. "We'll never really know the motive. I think they probably believed that their father was going to be a burden to them."

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The man to whom Linda initially confessed says Linda floated a similar justification.

"'I don't buy that euthanasia story,'" he says he told her. "'You killed your father. And this is insane.' And then I started asking her how she did it."

"He was complaining about going into an assisted living facility," the man recalls of that conversation. "They wanted him to go, they had it all set up, and he wouldn't go."

"She was saying he was old. He had cancer. He wasn't going to live long. They were just trying to hurry along his death."

Confession Followed Break-Up: 'Desperate Woman'

The man knew both women intimately. He'd met Mary Beth first and began a short-term sexual relationship with her. Mary Beth introduced him to Linda, he says, and he then began a longer-term sexual relationship with her, too.

He surmises Linda may have confessed to him as a way to explain her anguish after he said he was going to end their relationship. "I think she's sad and lonely and wanted to ... get it off her chest," he says. He described her as "a desperate woman trying to get a man back."

After hearing that confession, the man contacted the sheriff's office the next day.

"He was nervous to meet with us," says detective Nelson de Leon. "He was worried about getting in trouble with the recording."

De Leon and another detective, Michael Bailey, assured the man that he wasn't culpable — but also told him they couldn't use his unauthorized recording as evidence. So they fitted him with a wire and sent him to meet again with Linda that night as they listened in.

Days later, again wearing a wire, the man went to meet Mary Beth.

"Your sister told on you about what you guys did," the man says he told Mary Beth. "She looked like she was hit with a ton of bricks."

Mary Beth, he says, "spilled her guts and blamed Linda a lot. 'It was mostly Linda's idea,' and she went along with it. She said that from the beginning."

Both women were arrested on March 5, 2019. After reaching plea deals that avoided a trial, Linda was sentenced in March of this year, and Mary Beth the following month.

"It was very unconventional how this case came about," says prosecutor Koskinas. "We wouldn't have solved it without law enforcement. But [the man] initially brought it to our attention, so I commend him for that."

Says the man: "I'm not afraid of anything, but it was bone-chilling, having someone telling me they killed somebody. The ending of the life, hearing about how it happened. There's nothing like it. It's not a movie. It's real life."

He turned in the women because "you can't end somebody's life, period," he says. "It doesn't matter if they have five days, five months, five minutes, you're not the arbiter of when it's time for someone to die."

Left alone in an interrogation room, according to a transcript of their conversation, Linda said to Mary Beth, "I can't believe I had told him."

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