“I think my father, Rex Thomas, was murdered,” says Amanda, despite an autopsy that determined he died of natural causes.

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She says that her father was diagnosed with pulmonary fibrosis in August 2014 and went into the hospital in December, and by January 5, 2015, he was dead.

“The day after my father died, we found a note that he left for us in his Book of Mormon. The note, literally, said, ‘My primary care nurse is forcing me to take Fentanyl despite my oxygen saturation being at 72%. Fentanyl lowers oxygen saturation. I think she may be trying to kill me,’” claims Amanda. “My father said he thought someone was trying to kill him and six hours later, he was dead. I know my father was murdered and I want justice.”

WATCH: What Daughter Claims Father Wrote In Letter Hours Before He Died

Amanda’s mother says getting answers about her father’s death has consumed her daughter. “Before Rex died, she loved to go out, and Karaoke, and meet people and dance,” Sandy says. “She used to really want to get her degree, and she had goals. And then, all of a sudden, it was nothing.”

WATCH: Man Says Sister Has 'Put Herself In A Perpetual State Of Re-Living My Dad’s Death'

Amanda declares, “I won’t stop fighting for my father until I get all the answers.”

On Monday’s episode of Dr. Phil, does Dr. Phil tell Amanda he thinks she should keep up the fight, or that she has no case? And, hear what a cardiologist has to say about Rex’s death. Check here to see where you can watch.

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Why Woman Says Ex-Husband Wasn’t Murdered Even Though Daughter Claims He Was