A chilling story of marital infidelity and murder has been told in a Northern Ireland court as it emerged two partners accused of killing their respective partners in 1991 fooled the police for almost 20 years. They claimed their deaths had been part of a suicide pact.

The lovers Hazel Stewart and Colin Howell had kept their secret for nearly two decades.

In 2009 Howell contacted elders at his church and confessed to having poisoned his wife, Lesley, 31, and Trevor Buchan, 32. He killed them by pumping car fumes into their homes using a hosepipe. Last December he was sentenced to 21 years in prison.

However, his partner in crime, Hazel Stewart  denies the murders.

On the first day of evidence in the murder trial, in Coleraine Crown Court, she told the court that she had heard a struggle in the bedroom where her husband sleep just after Howell had enter the room to place the hosepipe.

According to reports, in January 2009, while being interviewed by police, she said "I didn't want to hear it, I put my hands over my ears, I didn't want to hear it, I was so scared…It was his plan…I didn't want to know."

The lawyer for the prosecution said "She stood feet away knowing her husband was struggling for his last breaths. She showed total and utter callous disregard for her husband and endorsed and encouraged exactly what Colin Howell was doing."

Stewart also told the court how she had given her husband a temazpem under her husband's instruction. She said "Colin gave me the tablets to give to him…If he hadn't taken it I would have had to give him something ... he had to have something in him to relax a bit. If Trevor hadn't taken it he [Howell] couldn't have gone on with it."

She told the court that Howell dressed Buchanan and put his body in the trunk of his car alongside his own wife who he had murdered earlier. He then drove the bodies to a nearby group of houses and stage-managed the suicide pact. Howell moved Buchanan's body to the driver's seat. He placed family photos in the trunk with his wife's body and placed a length vacuum piping from the exhaust pipe into the car. Before he left the crime scene he switched on the ignition.

The court also heard that shortly before Howell had murdered his wife he almost electrocuted his wife in the bath. A friend of his wife Margaret Topping told the court that he had dropped an electrical cable into the bath giving her a small electric shock.

Topping said "She told me almost laughing…She said it was so awful he could not have meant it. But she told me, so that I would know."

It is expected that Stewart's trial could last up to four weeks.

Colin Howell and Hazel Stewart