the story of Joanna Dennyhy; How a young loving girl turned into a serial killer

John Treanor will never forget the day he first met the fresh-faced teenager who would eventually become the mother of his two children.

Aged 21, and unemployed, he was taking his German Shepherd on an afternoon walk through a park near the home where  he lived with his parents in the Hertfordshire commuter village of Redbourn.

A dark-haired girl bounded over, patted the creature, and declared: ‘I really love dogs.

She introduced herself as Joanna Dennehy, a 15-year-old GCSE student at Roundwood Park School in nearby Harpenden.

They fell into conversation, and agreed to meet up again the following day to walk together. It was the summer of 1997, and the start of a rapidly-blossoming relationship. Despite their age gap, Treanor recalls being smitten by the gregarious, extrovert teenager.

 ‘She was quite normal: a little rebellious, perhaps, but nothing too out of the ordinary for a teenager.’ 

Today, the older, wiser, slightly world-weary Treanor looks back on that day with a mixture of sadness and regret.

The relationship it spawned would last 12 years, and produce two daughters - now aged 14 and seven - whom he of course adores.

But it ended in traumatic circumstances, after Dennehy became addicted to drink and drugs, pursued a string of tawdry affairs (with both men and women), and began physically and verbally abusing her family. 

After years of increasingly torrid abuse, Treanor walked out in 2009. The final straw, he recalls, was an argument which saw his drunken girlfriend reach into her knee-high leather boot and pull out a six-inch dagger.

She then plunged it into the living room floor, where it stuck, and yelled: ‘I wish I could kill someone!’

‘Jo was just angry. I don’t know what about. But that dagger was very, very scary,’ he recalls.

‘It was a proper one with a long, straight blade and decoration on the handle. Obviously, it really freaked me out.’

For his safety, and the safety of the children, he decided to disappear and set up home hundreds of miles away in Derbyshire, where he lives today with his daughters.

This week, a jury in Cambridge convicted two men of helping Dennehy carry out an unimaginably brutal, and at times random, killing spree, in which three men were stabbed to death and two more were seriously injured.