We used to call it hunting, we did it as a bit of a joke
In 2000, Duffy appeared at the Old Bailey as a witness against Mulcahy and gave detailed and graphic evidence over 14 days. It was the first time a highest-category prisoner had ever given evidence against an accomplice.
Mulcahy emerged as the chief perpetrator of the crimes and the first to decide that sexual stimulation was no longer enough of a thrill, so turning to murder. He was said by a former employee at a cab firm he worked at to despise women. "He liked women to be at the kitchen sink where they should be, or in bed", said Lola Barry, a controller at the cab firm.
She said Mulcahy had once crept up behind her in the office, "He actually got me round the neck, saying 'How does that feel - are you scared?'”
Mr Mark Dennis, prosecuting, told Mulcahy's trial, "As they fed their new-found predilections they treated their victims as objects rather than persons." It was "only a comparatively small step" between the violence of the rapes and the murders - and Mulcahy was the first to take it. "He was the instigator and prime mover in the murders, and the one for whom the sexual abuse had become insufficient to satisfy", said Mr Dennis.
Duffy, in the witness box catalogued their heinous campaign of rape and murder describing how the two friends would go out on 'hunting parties' in the 1980s searching for women. Duffy, used his knowledge of the rail network to target his victims and drag them into concealed areas where they could be attacked.
“We would have balaclavas and knives", Duffy claimed. "We used to call it hunting. We did it as a bit of a joke. A bit of a game."
Mulcahy protested his innocence but on 5 February 2001, was given three life sentences for murdering three women. He also received 24-year jail terms on each of seven counts of rape and 18 years each for five conspiracies to rape, to run concurrently.
The police believed that the two men were probably responsible for more deaths and sexual attacks and reinvestigated the 1980 murder of Jenny Ronaldson, 19, who was sexually assaulted, strangled and thrown in the Thames.
Apart from the level of ferocity associated with this case, the Duffy/Mulcahy casebook is one of the most significant criminal cases for its first use in Britain of 'Psychological Offender Profiling'.