Two sisters in Florida almost got away with the "perfect murder" of their elderly father until one of them confessed to the man they were both romantically involved with, authorities said.

Linda Roberts and Mary-Beth Tomaselli were arrested Tuesday and charged with first-degree murder, four years after their father, Anthony Tomaselli, 85, died in their Palm Harbor home.

Detectives began investigating Anthony Tomaselli's death as a homicide last month after a man dating both sisters called the sheriff's department and said Roberts, 62, had confessed to him that she and Mary-Beth Tomaselli, 63, "euthanized" their father because he was ill and would not go into an assisted living facility.

Deputies were called to the home March 6, 2015, for a report of a man not breathing and were told by Roberts and Mary-Beth Tomaselli that they awoke to find their father lying on a couch not breathing, Pinellas County Sheriff Bob Gualtieri said during a news conference Tuesday.

The sisters told deputies they performed CPR and called 911. Anthony, who had cancer, dementia and heart problems, was pronounced dead at the house, and it was initially determined that he died from natural causes, Gualtieri said.

Anthony Tomaselli Pinellas County Sheriff's Office

“There were no signs of foul play, no signs of criminal activity," he said. "Nothing to indicate anything other than Anthony’s death was of natural causes given his advanced age and medical condition.”

The boyfriend secretly recorded both women, who allegedly detailed how on the night of March 5, 2015, they gave their father a concoction of alcohol and sleeping pills to try and kill him, officials said.

Gualtieri said that Mary-Beth Tomaselli added too much alcohol to the mixture and diluted the sleeping pills, so it did not work. The sisters then allegedly tried suffocating their father with a pillow, and when that did not kill him, Roberts stuffed a rag down his throat while her sister held his nose and arms until he died.

Roberts, who is married, also gave her adult daughter, who was in the home at the time of the alleged murder, sleeping pills so she would not wake up and catch them in the act, Gualtieri said. In another recording taken by the boyfriend last week, Roberts confessed to also wanting to kill her husband, according to authorities.

After their father's death, the sisters and their brother sold their father's house and split a $120,000 profit. The brother was not involved in the murder, authorities said.

"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," Gualtieri said. “It’s terrible they put that much thought and effort into killing their dad."

Roberts and Mary-Beth Tomaselli are being held at the Pinellas County Jail.