For 24 years Josef Fritzl kept his daughter captive in a cellar. A hospital visit was his undoing.

Content warning: This story includes mentions of child sexual abuse that may be distressing to some readers.

It's one of the most chilling crime stories of modern times. 

For 24 years, Josef Fritzl abused his daughter, Elisabeth Fritzl, keeping her locked in a basement for much of that time. 

It all began in 1984, when Elisabeth was 18.

She says her father asked her to help him fix a door in the cellar at their home in Amstetten, Lower Austria. 

He had been building out the basement for a number of years, fit with layers of concrete and a heavy hinged door. A total of eight doors had to be opened before reaching the purpose-built cellar; it was effectively a dungeon. 

Fritzl kept his real plans for the cellar secret from his family. 

On that fateful day, while Elisabeth was down there, her father held a piece of cloth soaked in ether over her mouth and nose, drugging her.

He then held her captive so he could sexually abuse her. 

Fritzl told his wife, Rosemarie, and their other children, that Elisabeth had run away from home to join a religious cult. The family believed Fritzl, as Elisabeth had previously threatened to run away many times as a teenager. 

Fritzl had a history of violent behaviour. In 1967, he broke into the home of a 24-year-old nurse and sexually assaulted her, holding a knife to the victim's throat.

He was also named a suspect in an attempted rape that same year. Fritzl was arrested and served 12 months in prison for the 1967 attack. But in accordance with Austrian law, his criminal record was expunged after 15 years. He was no longer on the radar of officials.

Watch: The case unpacked on 60 Minutes Australia. Post continues below.

Over the next 24 years, Elisabeth was kept captive in the cellar. During winter it was damp and cold, and in summer the basement essentially turned into a sauna. 

Fritzl repeatedly threatened Elisabeth by saying: "If you do not do as I say, your treatment will get worse and you will not escape from the cellar anyway."

The sexual abuse continued and as a result, Elisabeth fell pregnant on numerous occasions, giving birth to seven babies. 

Three of the children stayed underground with their mother. One died shortly after birth, Fritzl disposing of the body via cremation.

The other three children were allowed to live upstairs with Fritzl and his wife Rosemarie. It is believed Rosemarie was duped throughout the entire ordeal, officials saying Fritzl "very plausibly" convinced his wife that the three kids had randomly shown up on their doorstep and needed help. 

Fritzl and Rosemarie then became foster parents to these three kids, and were granted permission by social workers and Austrian family services to look after them. These services saw and heard nothing to arouse suspicion. 

Then in 2008, Elisabeth found her chance for freedom.

Her 19-year-old daughter had become gravely ill, and Elisabeth managed to convince Fritzl to take her to hospital.

There he told staff Elisabeth had run away to a religious commune years earlier, only to return with her teenage daughters to live with him.

Medical staff were dubious about the story and alerted authorities. 

Later, Elisabeth was allowed to return to the hospital alone to see her daughter and authorities were there waiting. 

And that is when the then 42-year-old told police her story. 

Fritzl was arrested and charged with rape, false imprisonment, manslaughter by negligence, and incest. 

In March 2009, Fritzl was sentenced to life imprisonment without the possibility of parole for 15 years - which brings us to 2024. 

Now Fritzl is 88, and according to reports, could be released from prison.

Austrian media says Fritzl now has dementia and "no longer poses a danger to the public" according to the prison's psychiatrists. It's likely he will be granted conditional release, meaning he could be relocated into a care facility.

Josef Fritzl soon after his arrest and charges in 2008. Image: AAP.

Elisabeth managed to survive against the odds.

She chose to attend the second day of the criminal trial against her father. Reports say that she was in disguise while in the visitors' gallery to avoid media attention. Fritzl recognised his daughter though, and his lawyer later said that Fritzl "went pale and broke down". 

The following day in court, Fritzl changed his pleas to guilty on all charges. 

For some time after the escape, Elisabeth and her children were taken into care at a local clinic. In the years since they have received extensive psychological and medical treatment. 

In May 2008, a handmade poster created by Elisabeth, her children and her mother at the therapy facility was displayed in the Amstetten Town Centre. The message thanked local people for their support. 

"We, the whole family, would like to take the opportunity to thank all of you for sympathy at our fate," they wrote in their message. "Your compassion is helping us greatly to overcome these difficult times, and it shows us there also are good and honest people here who really care for us. We hope that soon there will be a time where we can find our way back into a normal life."

A mutual friend to Elisabeth and Rosemarie later remarked: "Rosemarie is a weak character who was as much a victim of Fritzl as everyone else." 

Elisabeth's children are now between the ages of 21 and 35. 

They live with Elisabeth in a tiny hamlet in the Austrian countryside, only known publicly as 'Village X'. Elisabeth was offered to be given a new name after the trial, and she accepted. Strict Austrian laws now prevent her identity from being revealed. 

The children reportedly sleep in rooms with doors permanently open in a bid to eliminate the trauma associated with their time in the cellar. 

They have also been provided with constant CCTV surveillance and security guard patrols - the Austrian government saying they wish to help the family heal in whatever way possible.

Elisabeth has also since found love. A year following her father's arrest, she met Thomas Wagner - an Austrian bodyguard who was assigned to protect her. The pair fell in love, and he then moved in with her family. 

If this has raised any issues for you, or if you just feel like you need to speak to someone, please call 1800 RESPECT (1800 737 732) – the national sexual assault, domestic and family violence counselling service.

If this brings up any issues for you, contact Bravehearts, an organisation dedicated to the prevention and treatment of child sexual abuse, on 1800 272 831.

Feature Image: AAP.