Warning: The following contains descriptions of violence.

When police officers Paul Finlayson and Tracy Cope responded to an emergency call that a woman had been struck by a car on England’s M6 motorway in May 2008, they expected a casualty. Chaos. Blood. Trauma. When they arrived, they found two women standing beside the carriageway, one calmly speaking to the first officer on the scene, the other smoking a cigarette.

Authorities huddled as they tried to determine what had happened. CCTV had captured 40-year-old Swedish identical twins Ursula and Sabina Eriksson as they walked down the centre of the motorway, climbed over the safety barrier and ran into traffic. One had flung herself straight into the side of a passing car.

A cameraman from the BBC reality television program Motorway Cops was also on the scene, and recorded as PCs Finlayson and Cope were briefed on the bizarre events. In the background, the sisters stood on the roadside, unfazed and seemingly uninjured. Then, without warning, Ursula bolted.

An officer grabbed desperately at her clothes, but she slipped free of her jacket and dove beneath the wheels of an oncoming semi-trailer. As police shouted in horror, Sabina followed her sister, straight into the path of a small hatchback.

“It literally threw her up into the air like a rag doll,” PC Cope later told 60 Minutes. “It’s shock. It’s horror. Disbelief.”

The camera kept rolling as the twins, both of whom had miraculously survived, fought against officers trying to treat them. Ursula, though crushed from the waist down, spat and wrestled, yelling, “I recognise you — I know you’re not real [police].” After 15 minutes, Sabina stirred back into consciousness, fought off PC Cope and tried to flee. “They’re trying to steal your organs,” she yelled. Bystanders helped police detain her, before paramedics arrived. Sabina was sedated, treated and both were rushed to hospital.

The events that day were only the beginning of what would become one of the UK’s most baffling criminal cases, and one of the most infamous suspected examples of a rare psychiatric disorder called folie à deux.

Also known as induced delusional disorder or ‘shared psychosis’, the term refers to cases in which a person’s delusional beliefs are adopted by others. It typically occurs between people in close relationships — siblings, couples, best friends, parents and their children — and lasts for a short amount of time. (Australia’s best-known example is that of the Tromp family, who were reported missing in 2016 when they embarked on paranoia-fulled road trip to NSW’s Blue Mountains.)

The Eriksson twins’ bizarre journey.

Though born and raised in Sweden, the Eriksson sisters wound up living on opposite sides of the world; Ursula in the USA, and Sabina in Ireland with her partner and two children.

Ursula came to visit her sister on May 16, 2008, and for reasons that remain unclear they travelled to Liverpool, England, without the knowledge of Sabina’s family. There, they boarded a bus to London, but were kicked off by the driver halfway through the journey due to their erratic behaviour.

Rather than catch the next bus they set off down the motorway on foot, leading to the bizarre incident on the M6.

Ursula was hospitalised for three months with severe crush injuries to her legs, and returned to the US once she was discharged.

Sabina, though, was cleared medically and released from hospital after just five hours and escorted away by police. On May 19, she pleaded guilty to trespass on the motorway and hitting a police officer, and was sentenced to a day in custody, which she'd already served.

The murder of Glenn Hollinshead.

Clutching a plastic bag of her belongings returned to her by police, Sabina left court and wandered the streets of Stoke-on-Trent, a county in the British Midlands. That evening, she struck up a conversation with local man Glenn Hollinshead, a 54-year-old welder and former member of the air-force who was out walking his dog with friend, Peter Molloy.

She told them she was searching for her sister, who was in hospital, and needed to find a hotel or bed and breakfast. A compassionate man, Glenn instead invited her to his place for a warm meal and offered her a place to sleep.

Once there, Sabina behaved in an increasingly erratic way. At one point, Peter later told The Staffordshire Sentinel, she handed them both cigarettes then snatched them back, claiming they could be poisoned.

"There were a million and one signs that something wasn't right, but there was absolutely no way I could have imagined what was going to happen to Glenn," he told the paper.

"Sabina was friendly, but I felt nervous about her, and I should have trusted my feelings."

Peter left that night. By the following evening, Sabina had stabbed Glenn to death with a kitchen knife.

CCTV footage captured her fleeing the scene, clutching a hammer and striking herself in the head. A passing motorist stopped and tried to wrestle it from her grip, but she struck him over the head with a roof tile she'd been carrying in her pocket.

Sabina then ran to nearby overpass, and leapt off. She survived the 12 metre fall to the road below, but suffered fractures to her legs and skull. Like her sister, she was hospitalised for three months.

During her eventual police interview, Sabina answered "No comment" to all questions. She had no history of mental illness, nor any previous criminal convictions.

Sabina pleaded guilty to manslaughter with diminished responsibility on September 2, 2009. The defence argued that she was the secondary sufferer of folie à deux, influenced by her twin.

She was sentenced to five years in prison, after the Luton Crown Court judge agreed she had low culpability for her crime due to mental illness: "It is not designed to reflect the grief the relatives have suffered or to measure the value of Mr Hollinshead's life," the judge said, according to BBC. "No sentence that I could pass could do that. It is a sentence which I hope fairly measures a truly tragic event."

For 24-hour mental health crisis support, please contact Lifeline on 13 11 14.