Colin Howell's second wife has described her former husband as a monster ahead of a TV drama about the killer dentist.
New York native Kyle Jorgensen said that for 10 years Howell forced her to keep it secret that he had murdered both his first wife Lesley and police officer Trevor Buchanan, the husband of his mistress, Hazel Stewart.
Howell told her not to utter a word for the sake of their children, or he would take his own life.
Ms Jorgensen, who now lives in Florida, told the Sunday Mirror that Howell had admitted the murders to her in 1998 when the couple were raising their seven-month-old son Erik.
He also told her that he had sexually assaulted his female patients while they were sedated.
She said she sat stunned at the kitchen table as Howell admitted his crimes.
"He abused patients, he is a compulsive liar, he was an adulterer in our marriage and murdered two people," she said. "Everyone thought he was this great Christian guy but they were so wrong. He was a monster."
Ms Jorgensen met Howell at church in Castlerock after moving to the UK after a violent three-year marriage to her first husband.
The story behind the 1991 murders will be told in a four-part TV drama starring Co Antrim actor James Nesbitt.
The Secret tells the dramatic story of how the Ballymoney dentist and lay preacher gassed his wife and Stewart's husband, and made their deaths look like a suicide pact.
The bodies of mum-of-four Lesley and dad-of-two Trevor were found in a car filled with fumes in Castlerock.
In 2009 Ms Jorgensen convinced Howell to confess and the terrible truth began to emerge.
In 2011, Howell was later sentenced to a minimum 21 years for the double murders. Hazel Stewart, who got married again, is spending 18 years in prison for her role in the killings.
The Secret begins on ITV on April 29