A probe into the death of the former husband of Lori Vallow, who is under investigation in connection with the deaths of her two children, found that he died of natural causes, authorities said. 

The Phoenix Police Department launched a review into Joseph Ryan's death after Vallow was allegedly heard on an October 2018 audio recording saying "I was going to murder him."

Ryan died in April 2018. A medical examiner determined the cause of death was a heart attack. 

REMAINS FOUND ON IDAHO PROPERTY ARE LORI VALLOW'S MISSING CHILDREN, FAMILY SAYS; CHAD DAYBELL CHARGED

Lori Vallow glances at the camera during her March 6, 2020 hearing in Rexburg, Idaho. At her right is defense attorney Edwina Elcox. She is charged with felony child abandonment after her two children went missing nearly six months ago had her bond reduced to $1 million by an Idaho judge on Friday. Phoenix authorities said the death of her third husband, Joseph Ryan, was natural causes. (John Roark/The Idaho Post-Register via AP, Pool) (AP)

"There is no foul play suspected," Phoenix police Sgt. Mercedes Fortune told Fox News. "There is no criminality associated with this investigation."

The recording was taken by Ryan's sister, Annie Cushing. It reportedly features Vallow saying she wanted to kill Ryan because of their bitter custody battle over their daughter, Tylee Ryan.

"He was constantly trying to get custody of my 3-year-old daughter," she allegedly said. "If he comes against you once, if he comes against you twice, if he comes against you three times then you can kill him, it says it in the scriptures." 

Vallow's fourth husband, Charles Vallow, was fatally shot in July 2019 by her brother Alex Cox. Cox, who claimed he killed his brother-in-law in self-defense, died in December of that year from natural causes. 

Authorities are reportedly looking into Lori Vallow for Charles Vallow's death. 

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Vallow and her current husband, Chad Daybell, are charged in Idaho in connection with concealment and other crimes related to the deaths of her children, 7-year-old Joshua "J.J." Vallow and 17-year-old Ryan. Their bodies were found in June on Daybell's property. 

Authorities are also looking into the death of Daybell's deceased wife, Tammy Daybell. She died weeks before he married Vallow.