'Possible' That Utah Doc's Wife Drugged and Drowned, Medical Examiner Says

Medical examiner says he changed cause of death due to possible role of drugs.

Oct. 31, 2013— -- Utah's chief medical examiner testified today that though a cause of death for Michele MacNeill was never determined, the prosecution's theory that she was drowned was plausible.

When asked by prosecutors if it was possible MacNeill died after being drugged and held down in a bathtub full of water, Utah State Chief Medical Examiner Dr. Todd Grey said it was a possibility.

MacNeill's cause of death was changed by Grey from "natural" to "undeterminable," he said, due to the possibility that drugs could have played a role in her death.

Despite the change, Grey told the court, "I did not feel I could reach a conclusion of homicide."

Dr. Martin MacNeill, 57, is on trial for the April 11, 2007, death of his wife, Michele MacNeill, 50. Prosecutors allege MacNeill persuaded his wife to have plastic surgery so he could dope her up during her recovery and then drown her -- all so he could pursue a relationship with a mistress, Gypsy Willis.

MacNeill's defense lawyers said heart problems were a contributing factor in the death of the mother of eight.

Dr. Maureen Frikke, a former assistant medical examiner who died in 2008, certified that MacNeill died of cardiovascular disease, high blood pressure and inflammation of the heart. That report was amended after Grey reviewed the case after Frikke's death.

Mistress Said Doctor Told Her Secret During 'Pillow Talk'

On Wednesday, one of two women who have testified that they had a sexual relationship with Dr. MacNeill while his wife was still alive, said the doctor once explained to her during "pillow talk" how he could induce a heart attack in someone while making it appear natural.

Anna Osborne Walthall testified that she began an affair with Dr. Martin MacNeill in 2005, when he was a consultant at a laser hair removal clinic she briefly operated in Utah.

Walthall said the doctor volunteered to serve as a "liaison" between her and her estranged husband during their divorce. Their relationship turned sexual in March 2005, she said, and ended six months later when Walthall moved out of town.

She said during a "pillow talk" moment she remembered the doctor telling her there was a natural way to induce someone to have a heart attack.

"There's something you can give someone that's natural that's a heart attack that's not detectable after they have a heart attack," she quoted MacNeill as saying.

The doctor's defense attorneys tried to discredit Walthall by mentioning she had been diagnosed with disassociative identity disorder.

They also pointed out that Walthall had emailed investigators with what she believed were evidentiary tips found in her dreams, asking them if there was anything about a "white Toyota" or if the autopsy said anything about Michele MacNeill's toes.

"You know I was processing and sometimes I just give too much information," Walthall said when the emails were brought up in court.

Doctor's Daughter Confronted Him About Over-Medicating Mom

In the days before Michele MacNeill was found lifeless in the bathtub, one of her eldest daughters, Alexis Somers, confronted her dad about over-medicating her mom as she recovered from plastic surgery.

After Alexis Somers found her mother "very sedated" one day after she came home from the hospital on April 4, 2007, Somers, who was then a medical student, said she told her father, Dr. Martin MacNeill, "I'm taking over."

Somers testified for the second time during her father's murder trial on Wednesday, which is in its third of five scheduled weeks in Provo, Utah. She said her mother was a reluctant plastic surgery patient who didn't like taking all of the drugs her physician husband insisted she take during her recovery.

After taking a cocktail of pills that left her sedated, Somers said her mother told her she didn't want MacNeill to dispense her medication anymore since she couldn't see through the bandages on her face.

"She actually had me take out every single pill from the pill bottles and she wanted to feel what the pills feel like in her fingers, so if my dad tried to give her anything, she would know what he was giving her," Somers said.

On April 10, 2007, Somers said she left her family's home in Pleasant Grove, Utah, to go back to medical school in Henderson, Nev.

Somers told the court her mother "was feeling really well" when she left.

The next day, the mother of eight was found by her 6-year-old daughter unconscious in a bathtub full of brown water. Less than two hours later, Michele MacNeill was dead.

Watch the Full '20/20' Report: Life of Lies

Daughter Saw Nanny Go to Dad's Room

Vanessa MacNeill, one of MacNeill's daughters, testified that she participated on a family panel to hire a new nanny days after her mother died. The only candidate her father presented, she said, was Willis.

Utah Doc's Youngest Daughter Recalls Finding Mom Submerged in 'Brown' Water

Sabrina MacNeill, 19, said Willis moved in shortly after her mother's death, however she never recalled Willis doing any typical nanny duties.

"She didn't do much. She made spaghetti once and that was the only time she ever cooked," Sabrina MacNeill said.

Even though Willis had a room downstairs at the family's home, Sabrina MacNeill said she noticed the nanny would go upstairs to her father's room at night and that the door would then be closed.

"I remember staying up at night and [thinking], 'What in the world? I thought she was our nanny. Why is she up in dad's room?'" she said.

The relationship between MacNeill and Willis moved full-speed ahead, and by late June, the couple were looking for wedding rings online, prosecutors noted. During a trip to Wyoming in July, just months after his wife died, MacNeill proposed to Willis at a restaurant with a four-and-a-half carat diamond ring.

Willis' mother, who testified Tuesday, said the family was "giddy" to have MacNeill, who is both a doctor and lawyer, in the family.

"Martin MacNeill is a very impressive person. He's tall, he has a bright white smile. He holds the title of doctor and also attorney," Vicki Willis said. "We were pleased, of course. Who wouldn't be?"