Sisters allegedly killed elderly dad in 'perfect murder' plot then covered it up for years: Sheriff Linda Roberts and Mary-Beth Tomaselli allegedly killed Anthony Tomaselli, 85.

Two sisters are accused of killing their 85-year-old father in a chilling, premeditated plot that was uncovered four years later, thanks to a man, who had a sexual relationship with both sisters and came forward, authorities said.

Linda Roberts, 61, and Mary-Beth Tomaselli, 63, were arrested and charged with first-degree murder for the 2015 death of their father, Anthony Tomaselli, the Pinellas County Sheriff's Office said in a statement on Tuesday.

"This is in some respects — as we sometimes call these things — it's the perfect murder, because there was absolutely no sign of struggle, no sign of foul play," Pinellas County Sheriff Bob Gualtieri told reporters Tuesday. "He had cancer, he had dementia, he was seriously ill ... They could have easily gotten away with it."

The alleged crime took place on March 6, 2015, when the sisters were at their father's Palm Harbor, Florida, home. The sisters called 911 for help and paramedics rushed to the scene and tried to do CPR on Anthony Tomaselli, but he was declared dead, according to the sheriff's office.

At the time "there did not appear to be any foul play or criminal activity," the sheriff's office said.

Anthony Tomaselli had an "extensive medical history and when contact was made with his primary care physician, he agreed to sign the death certificate," the sheriff's office said. "The physician listed the cause of death as natural due to Tomaselli's advanced age and medical history."

But this year, on Feb. 13, a man, who had a sexual relationship with both sisters, contacted authorities to say he had a recording of Linda Roberts confessing, Gualtieri said.

The man then helped authorities obtain recordings from both women about the crime, Gualtieri said.

The women allegedly said on the recordings that they first plotted to kill their father with alcohol and sleeping pills, but that didn't work, and they allegedly tried to suffocate him with a pillow, according to the sheriff's office.

"When that did not work, they stuffed a rag down his throat, pinched his nose, and held his arms down until he stopped breathing and died," according to the sheriff's office.

Mary-Beth Tomaselli allegedly said her adult daughter was at the home at the time so she gave her sleeping pills so she wouldn't see what happened to her grandfather, authorities said.

The sisters allegedly said they "faked" finding their father dead and staged giving him CPR and calling 911 for help, authorities said.

The sisters allegedly said "they 'euthanized' their father" in a "premeditated" killing "because he was going to die within a couple months and would not live in an Assisted Living Facility," the sheriff's office said.

After Anthony Tomaselli's death, his daughters sold his home and split the money with their brother, who was not involved in the alleged crime, authorities said.

The sisters admitted to the allegations when they were arrested Tuesday — a day before their father's death anniversary, according to the sheriff.

The sheriff praised the man, who allegedly recorded the sisters, for coming forward.

Roberts and Tomaselli are expected to get arraigned for the charges on Wednesday.

Neither defendant has entered a plea. Attorney information was not immediately available.

ABC News' Ahmad Hemingway contributed to this report.