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PALM HARBOR — Four years ago, Anthony Tomaselli stopped breathing. His two grown daughters tried to resuscitate him until paramedics arrived. His own doctor ruled that the 85-year-old man died of natural causes due to his age and health problems.

Now deputies say it was murder.

"It was the 'perfect' murder," Pinellas Sheriff Bob Gualtieri said Tuesday.

Tomaselli's daughters actually conspired to kill him in 2015, the sheriff said, and they almost got away with it — until both became romantically involved with the same man.

That led deputies on Tuesday to arrest sisters Mary-Beth Tomaselli, 63, and Linda Roberts, 61. Each faces a charge of first-degree murder.

At a news conference later that day, Gualtieri explained how they almost pulled it off — and how their alleged scheme unraveled:

A man called the agency on Feb. 13. The man, whose name was not released by the agency, told deputies that a women he was romantically involved with confessed to planning and murdering her father along with her sister.

He met Mary-Beth Tomaselli at a bar in August, the sheriff said, and they became intimate. Sometime later, she introduced him to her sister, Roberts. He then became romantically involved with her as well, though the sheriff did not elaborate.

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The man told detectives Roberts acted "odd," Gualtieri said.

"It was clear something was troubling her," the sheriff said.

Then, on Feb. 12, Roberts called the unidentified man to her home, the sheriff said, where she admitted to "euthanizing" her father.

The man took out his cell phone and started recording audio and video of their conversation, In it, Gualtieri said Roberts detailed what actually happened on March 5, 2015, the day the sisters are accused of carrying out their plot.

The two sisters knew their father's days were numbered. He had made it clear he didn't want to be put in an assisted living facility. So they decided to take matters into their own hands: They plotted to make it look like he died in his sleep at his home at 530 Natingham Close in Palm Harbor.

Gualtieri said the women gave their father an alcoholic beverage laced with an "excessive" amount of sleeping pills, hoping it would kill him. But Mary-Beth Tomaselli put too much alcohol in the drink, the sheriff said, diluting the pills and rendering them ineffective.

Their father lay on the couch, still moving, with labored breathing. So his daughters then tried to suffocate him with a pillow, Gualtieri said.

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And when that didn't work, they held down his arms, grabbed a rag, shoved it down his throat and pinched his nose, the sheriff said.

Finally, their father stopped breathing.

Throughout the incident, Mary-Beth Tomaselli's grown daughter was upstairs asleep. Her mother had also given her sleeping pills so that she wouldn't wake up during their scheme.

The next morning, March 6, 2015, paramedics raced to the home and, after failing to revive him, pronounced Anthony Tomaselli dead at 5:50 a.m.

Deputies arrived soon afterward and interviewed the two daughters separately. Both said the same thing: their father fell asleep on the couch and when they went to go check on him, he wasn't breathing. So, they performed CPR and called 911.

Deputies learned about the father's lengthy medical history. His longtime doctor agreed to declare the 85-year-old had died a natural death and sign the death certificate.

But the deputies had been misled. The daughters now say they they "faked" the whole story. They staged finding his body, performing CPR and calling 911, deputies say.

They got away with it, the sheriff said, because no autopsy was performed due to their father's age and health history.

"Every single day we respond to natural death calls and if there is no evidence of foul play, no suspected criminal activity, no reason to believe anything other than a natural death, none of that is checked," Gualtieri said. "When you have a guy who is 85-years-old and he has cancer, he has dementia and he obviously has heart problems, he has a pacemaker ..."

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After their father's death, Mary-Beth Tomaselli, Linda Roberts and their brother, who deputies said had no role in the plot, sold their father's Palm Harbor home.

The sheriff said the three siblings split the $120,000 profit.

When deputies questioned the daughters, the sheriff said, both confessed. He said Mary-Beth Tomaselli, in particular, "spilled her guts."

The sisters used words such as "premeditated" and "euthanized" several times in their interviews with investigators and in the recording the unidentified man provided, Gaultieri said.

"They clearly knew what they were doing," the sheriff said.

Both sisters were booked into the Pinellas County jail.

Contact McKenna Oxenden at moxenden@tampabay.com. Follow @mack_oxenden.