Image copyright Cambs Police

After killing her third victim, Joanne Dennehy phoned her friend and sang the Britney Spears track "Oops I Did It Again" down the line. Only she knows for sure why she embarked on her killing spree but police believe the best explanation is she was simply having "fun".

When Dennehy was caught - after two days on the run - she was calm and silent, sitting in the passenger seat of a green Vauxhall Astra in The Oval area of Hereford.

Outside were armed officers from West Mercia Police, being taunted by a 7ft 3in man - Gary Stretch - who told them: "I suppose I'm Britain's 'most wanted'." He was not. That dubious honour belonged to Dennehy.

She just stared straight through me Robin Bereza, Stabbed by Dennehy in Hereford

Her arrest brought an end to a cross-country spree of violence that had left three men dead and two wounded. All of them had been stabbed repeatedly by Dennehy with a 3in (9cm) pocket knife.

The mother-of-two from Peterborough had bragged to friends while on the run that she and Stretch were a modern day version of Bonnie and Clyde.

It is one of the few clues she has offered into her mindset. Police have struggled to find out why a woman who had rarely been in trouble - despite being heavily involved with drink and drugs - became one of Britain's most notorious killers.

The survivors and families of the murdered want to know why. But Dennehy, known as Star because of the tattoo on her cheek, has refused to shed any light on that.

Nothing in her criminal record of what police call "low-level offending" would have foretold the spilling of blood to come.

Image copyright BBC/Microsoft

1. Dogsthorpe, 19 March - Lukasz Slaboszewski murdered by Dennehy in a house owned by Kevin Lee. On 29 March , Dennehy murdered Kevin Lee in the same property

- Lukasz Slaboszewski murdered by Dennehy in a house owned by Kevin Lee. On , Dennehy murdered Kevin Lee in the same property 2. Yaxley, 29 March - Kevin Lee's car found burnt out

- Kevin Lee's car found burnt out 3. Bifield, 29 March - John Chapman, a housemate of Dennehy, is murdered

- John Chapman, a housemate of Dennehy, is murdered 4. Newborough, 30 March - Body of Kevin Lee discovered dumped

- Body of Kevin Lee discovered dumped 5. Thorney Dyke, 3 April - Bodies of Lukasz Slaboszewski and John Chapman found

Image copyright BBC/Microsoft

Lukasz Slaboszewski, described as the "joker in the family" by his sister Magda Skrzypczak, was the first to be murdered.

According to Det Ch Insp Martin Brunning, of Cambridgeshire Police, the 31-year-old thought he was about to have sex with Dennehy. He was wrong.

The murder victims Image copyright Cambs Police Lukasz Slaboszewski moved to the UK from Nowa Sol, Poland, in 2005 to work at the DHL warehouse in Peterborough. At the time of his death, he was prescribed methadone as a substitute for his heroin addiction. He was last seen alive leaving his home in Lincoln Road, Peterborough, on 19 March having met Dennehy for the first time the previous day. He told friends he had found an "English girlfriend". Kevin Lee lived in Fletton, Peterborough, with his wife Christina and their two children. He ran the Quick Let property firm, which specialised in providing bedsits to people of "limited resources". Business partner Paul Creed said Dennehy approached them seeking accommodation. Mr Creed had been reluctant to house her but Mr Lee wanted to "give her a chance". Originally from Leeds, John Chapman served with the Royal Navy during the Falkland's War. Jurors heard he had fallen on hard times. He was an alcoholic but known as a "friendly drunk". Mr Chapman lived in the same set of bedsits as Dennehy in Bifield, Orton Goldhay. He described Dennehy, who had been employed as an "enforcer" by Mr Lee, as the "man woman" and friends were concerned for his safety as she attempted to force him to move out, the court heard.

Although exactly what happened between them in the lounge area of a house in Rolleston Garth, Peterborough, may never be known, he ended up being repeatedly stabbed in the chest and dumped in a wheelie bin.

He is thought to have been murdered on 19 March, although his body was not discovered until days later.

Kevin Lee and Mr Chapman were murdered 10 days later on 29 March - Good Friday.

Mr Lee, 49, of Fletton, ran a lettings business in Peterborough and Dennehy looked after his properties in return for rent-free accommodation at his house in Bifield, where Mr Chapman and Leslie Layton also lived.

"Her work was her rent," said Det Ch Insp Brunning. But their relationship went further, at least in Mr Lee's eyes.

On the day he was murdered, Mr Lee bought four CDs at HMV in Peterborough before heading to Rollerston Garth to see Dennehy. Two of the CDs were for his wife, Cristina. The other two - one featuring Bobby Womack, the other by the band The XX - were for Dennehy.

What he might not have known was that Dennehy was furious with him. Before the killing, Stretch told his housemate Carla White how "Mr Lee hadn't paid Dennehy for work she had done on the house and she was getting stressed and he was harassing her".

"He said 'she will kill him'," Ms White said.

"I said 'don't be silly, she would not do anything like that'."

But she did. When Mr Lee's body was found by a farmer in a ditch near Newborough, he was dressed in a black sequin dress with his buttocks exposed.

This was a form of "final humiliation", a jury was told. Dennehy, as consultant forensic psychiatrist Dr Frank Farnham found when he assessed her after her arrest at HMP Bronzefield in Surrey, has the condition paraphilia sadomasochism.

Sadomasochism is a preference for sexual activity which involves the infliction of pain or humiliation or bondage.

Media playback is unsupported on your device Media caption Joanne Dennehy shown on CCTV at Green Lane Stores in Hereford moments before she attacked two strangers with a knife

Mr Lee, it seems, was aware of Dennehy's sexual interests. Before his death, Mr Lee had told a friend that Dennehy had wanted to "dress me up and rape me".

After murdering Mr Lee, Dennehy stabbed John Chapman - described by his family as "a loving brother, brother in law and uncle" - three times in the chest.

The likeliest reason, said Det Ch Insp Brunning, was the 57-year-old had seen Dennehy in the bath and, so the theory goes, laid his eyes upon her body for a little too long.

Image copyright Cambridgeshire Police Image caption Gary Stretch had warned a housemate that Dennehy would kill Mr Lee over money she felt he owed her

Having murdered him, Dennehy rang her long-time friend Stretch and sang the Britney Spears song down the line.

The next morning - Easter Sunday - Stretch and Dennehy went on the run with a duvet in the back of the Vauxhall, heading up to Norfolk to see Georgina Page, an old friend of Dennehy, before heading west, via Worcester, to Herefordshire.

Sitting in the back of the car was Mark Lloyd, who joined them in Kington after Stretch and Dennehy had carried out a burglary.

He said he was so scared of Dennehy that he had no choice but to join them.

"She wanted to be like Bonnie and Clyde," said Mr Lloyd. "She wanted nine victims."

Mr Lloyd, who said Dennehy was "flirting" with him, described how Dennehy told Stretch: "I want my fun. I need you to get my fun."

This "fun" was a euphemism for hunting men and stabbing them.

Image copyright Cambridgeshire Police Image caption Dennehy used this lock knife in all three murders and two attempted murders

In Hereford, Stretch drove around the town while the pair searched for victims. Mr Lloyd said she made it clear she did not want to kill a woman, especially not a woman with children.

At about 15:40, Dennehy left the car and approached Robin Bereza, a 64-year-old man who was out walking his dog in the Hunderton area of town. She stabbed him twice with the pocket knife - once in the chest and once in the shoulder area.

"I thought she was going to mug him but then it twigged on me. I thought 'you just want blood'," said Mr Lloyd.

Mr Bereza, a retired fire service employee, did not immediately realise he was being attacked.

Catching Dennehy and Stretch: The importance of mobile telephone data Use of mobile telephone data was crucial in piecing together the movements of Dennehy and Stretch. One of the key pieces of evidence was mobile phone data which put Dennehy in Yaxley when Mr Lee's Ford Mondeo was torched. Ironically, that data only came into existence because Kevin Lee's wife had become suspicious about her husband's activities and concerned as to his whereabouts on the day he was murdered. A number repeatedly appeared on his mobile telephone statements. The number listed was Dennehy's - although Mr Lee's wife did not know that at the time. She rang the number and it was that call which the police say proved Dennehy was "in the vicinity of the car fire at the time".

During the trial, he told the jury: "I felt a blow to my right shoulder. I turned around and saw this lady, she just stared straight through me.

"I kicked her and made contact. It had no impact on her. She just came straight towards me.

"I ran into the road. I put my hand to my jacket and saw all this blood.

"She tried to come for me again, I kicked her again; she still didn't react."

After the attack, Mr Lloyd said Dennehy got back into the car smiling. Mr Bereza remembers seeing her get back into a car with a large man.

Dennehy kissed Stretch on the cheek before saying "thanks". She said the next victim had to have a dog, according to Mr Lloyd.

And within 10 minutes, she and Stretch found another target - 56-year-old John Rogers, another man out walking his dog.

He was stabbed 40 times in his arm, chest, stomach and back and left for dead in the street.

The first contact, he said, felt like a "punch in the back". He assumed it was a friend or a neighbour just "messing about".

"I turned around and saw the woman who stabbed me just standing there," Mr Rogers said. "She started stabbing me in the chest."

He asked Dennehy: "What's this all about?"

She told him he was bleeding before saying: "I better do some more."

"I said 'just leave me alone please, please can you leave me alone', but she didn't. She didn't seem to be showing any emotion.

Image caption Mark Lloyd said Dennehy flirted with him while he sat in the back of the car on the drive from Kington to Hereford, where the two attempted murders took place

"She didn't seem to be enjoying herself. She just seemed like she was going about business."

Mr Rogers said he fell to the ground but the attack continued. "I was just waiting for it to stop. There was loads and loads of blood on the floor on the ground.

"As I lay there I thought 'this is where I'm going to die'."

Before members of the public begin gathering around him in a desperate attempt to save his life, Dennehy left the scene, taking Mr Rogers' lurcher dog with her.

Det Insp David Williams, of West Mercia Police, said the actions of those passers-by undoubtedly saved Mr Rogers' life.

Detectives believe Dennehy targeted Mr Bereza and Mr Rogers simply for sport.

Media playback is unsupported on your device Media caption John Rogers was stabbed more than 30 times by Joanne Dennehy

Det Insp Williams said: "She was attacking for fun. Robbery was not a motive. They were driving around and carrying out random but deliberate attacks."

Mr Rogers was left with "significant nerve damage and mental trauma", he added.

Asked what the trigger was for the murderous attacks, Det Ch Insp Brunning said: "I have no explanation for you."

And asked if he had ever seen a case like this, he shook his head and said: "No, have you?"

One of those who is aware of similar cases, however, is Elie Godsi, a consultant clinical psychologist.

Dennehy, says Mr Godsi, was a walking, talking "powder keg" waiting to go off. That the touch paper for her violence was sexual - in this case with Mr Slaboszewski - was no coincidence.

"Working with hundreds and hundreds of violent people, the same things come up time and time again," he said.

Image caption Det Ch Insp Martin Brunning says the trigger for the killings may never be known

"Dennehy has a serious history of self-harm. Why was she self-harming? It is almost always because they have a history of abuse. We are very likely to have here a woman with a history of serious sexual abuse.

"She will be somebody whose experience of sexual relationships is about being brutalised.

"When that happens to young men, they very quickly become violent. It tends to take women much more time to get to the tipping point. It is much more likely for them to self harm and have mental health problems.

"Men act out their distress, women feel their distress."

Mr Godsi said on the day Dennehy met Mr Slaboszewski, she "flipped" and then "externalised" her feelings about sex and brutality.

"She's just turning the tables," he said.

But despite Dennehy using the term "fun" to describe the attempted murders in Hereford, they were far from that, in Mr Godsi's opinion.

Rather, they were what he describes as the culmination of a "whole combination of events".

Image copyright BBC/Laurence Cawley Image caption Det Insp David Williams, of West Mercia Police, said the attacks were "random but deliberate"

Mr Bereza and Mr Rogers were the innocent victims of Dennehy's "displaced anger" which had been set loose during the first murder.

"She knifed herself, and then other people," he added.

Is it possible, though, that Dennehy will ever admit why she carried out the attacks?

Dr Elizabeth Yardley, senior lecturer at Birmingham City University's centre for applied criminology, believes it is likely she will not.

Dennehy's guilty pleas - unexpected even by her own defence team - have robbed her victims' families of an explanation and could be interpreted as an attempt to retain control.

"This is a woman who clearly likes messing with people so to throw her own legal team into chaos would have given her a sense of satisfaction," Dr Yardley said.

"But if it had gone to trial and she had given evidence, we wouldn't necessarily be any closer to knowing the truth of why she did what she did.

"Serial killers do not give the truth away very easily - it gives them power that they can continue to use long after they are convicted."