A scheming dentist murdered his wife and his lover's policeman husband by poisoning them with car exhaust fumes as they slept, a court heard.
Colin Howell then fooled investigating police by stage-managing a scene to make it look as if they had died in a suicide pact, Belfast Crown Court heard.
He kept the secret for almost 20 years before confessing to detectives.
Howell, 51, first killed his wife Lesley and later Constable Trevor Buchanan at their homes in Coleraine, Co Londonderry, in May 1991 when he attached part of a baby's feeding bottle to a garden hose to poison them with carbon monoxide fumes from a car.
Details of the double murder were revealed for the first time at a pre-sentence hearing for Howell, who has pleaded guilty to the murders.
Crown lawyer Kieran Murphy told Mr Justice Anthony Hart: "This was a meticulous and devious plan, and pre-meditated in a manner that might be described as professional."
Howell, of Glebe Road, Castlerock, Co Londonderry, has already been sentenced to life imprisonment and will be told on Friday the minimum term he will serve.
His former lover, mother-of-two Hazel Stewart, who remarried after Constable Buchanan's death, has pleaded not guilty and her trial, which is due to begin in early February, is expected to last up to a month.
The court also heard that two years after he remarried, Howell told his second wife Kyle about the killings but his admission was not disclosed until January last year, when he confessed all to his church elders.
Howell and Constable Buchanan's wife had resumed their affair when the dentist decided to carry out the murders late on May 18 and the early hours of May 19.