On 19 April 2008, Kerstin, Elisabeth's eldest daughter, fell unconscious, and Fritzl agreed to seek medical attention.

On 19 April 2008, Kerstin, Elisabeth's eldest daughter, fell unconscious, and Fritzl agreed to seek medical attention. Elisabeth helped Fritzl carry Kerstin out of the dungeon and saw the outside world for the first time in 24 years. She was then made to return to the dungeon where she would remain for a final week. Kerstin was taken by ambulance to a local hospital (Landesklinikum Amstetten) and admitted in serious condition with life-threatening kidney failure. Fritzl later arrived at the hospital claiming to have found a note written by Kerstin's mother. He discussed Kerstin's condition and the note with Dr. Albert Reiter.] Medical staff found aspects of the story to be puzzling and alerted the police on April 21, who then broadcast an appeal via public media for the missing mother to come forward and provide additional information about Kerstin's medical history. The police then reopened the case file on a missing Elisabeth. Fritzl repeated his story about Elisabeth being in a cult, and presented what, he claimed, was the "most recent letter" from her, dated January 2008. It was posted from the town of Kematen, in order to create a false lead.
The police contacted Manfred Wohlfahrt, a church officer responsible for collecting information on religious cults. Wohlfahrt raised doubts about the existence of the cult. He noted that Elisabeth's letters seemed dictated and oddly written. The news covered some of these issues and Elisabeth watched the story, as it progressed, on the television in the cellar. She pleaded with her father to be taken to the hospital. On April 26, Fritzl released Elisabeth from the cellar along with her sons Stefan and Felix, bringing them upstairs. Fritzl told his wife that Elisabeth had decided to come back after a 24-year absence. Governor Lenze told ORF that Fritzl had telephoned him and thanked him and the social services for looking after his family during his granddaughter Kerstin's illness. Fritzl and Elisabeth went to the hospital where Kerstin was being treated on April 26, 2008. Following a confidential tipoff by Dr. Albert Reiter reporting that the two were at the hospital, the police detained Elisabeth and her father on the hospital grounds and took them to a police station for further interrogation.
Elisabeth did not provide police with more details until they were able to reassure her that she would be safe from her father and her children would be looked after. In the space of two hours, she told the story of her 24 years in captivity. Shortly after midnight, police officers completed the three pages of minutes of the interrogation. Fritzl was arrested on suspicion of serious crimes against family members, facing possible charges of false imprisonment, rape, manslaughter by negligence, and incest. During the night of 26-27 April, Elisabeth, her five children and her mother Rosemarie were taken into care.
Fritzl confessed on 28 April to having imprisoned his daughter for 24 years and having fathered her seven children. Police said Fritzl had told investigators how to enter the basement prison through a small hidden door, opened by a secret keyless entry code. Fritzl's wife, Rosemarie, had, apparently, been unaware of what had been happening to Elisabeth. It is believed she assumed, due to the letters in her handwriting, that her daughter had run away from home to join a religious cult.
On 29 April, it was announced that DNA evidence had confirmed that Fritzl is indeed the biological father of all of his daughter's children.
Fritzl's defence lawyer, Rudolf Mayer, said that, although the DNA test proved incest, evidence was still needed for the other allegations: "The allegations of rape and enslaving people have not been proven. We need to reassess the confessions made so far."
In their daily press conference, Austrian police said on 1 May that Fritzl had forced Elisabeth to write a letter the previous year indicating that he may have been planning to release her and the children. In it, she wrote that she wanted to come home but "it's not possible yet". Police believe Fritzl intended to pretend he had rescued his daughter from her fictional cult. In the same press conference, police spokesman Franz Polzer said the investigation would probably last a few months, as police planned on interviewing at least 100 people who had lived as tenants in the Fritzl apartment building in the previous 24 years.
Investigators have only been allowed to work in the cellar for an hour at any given time, due to the lack of oxygen.
Dungeon

The Fritzl property in Amstetten consists of a building dating from around 1890 and a newer building, which was added after 1978, when Fritzl applied for a building permit for an "extension with basement". In 1983, building inspectors visited the site and verified that the new extension had been built according to the dimensions specified on the building permit. Unknown to them, however, Fritzl had illegally created additional room by excavating space for a much larger basement and concealed it by erecting walls. Around 1981 or 1982, according to his statement,[12] he started to turn this hidden cellar into a prison cell and installed a washbasin, a toilet, a bed, a hot plate and a refrigerator. In 1993, he added more space by creating a passageway to a pre-existing basement area under the old part of the property, which no one knew of apart from him.
The concealed cellar was soundproofed and consisted of a 5 m (5.5 yd) long corridor, a storage area, and three small open cells, connected by narrow passageways: a basic cooking area and bathroom facilities, followed by two sleeping areas, which were equipped with two beds each. It covered an area of approximately 55 m² (600 sq ft). The ceilings were no more than 1.70 m (5.6 ft) high.
The hidden cellar had two access points: a hinged door that weighed 500 kg (1,100 lb) which is thought to have become unusable over the years because of its weight, and a metal door, reinforced with concrete and on steel rails that weighed 300 kg (650 lb) and measured 1 m (3.3 ft) high and 60 cm (2 ft) wide. It was located behind a shelf in Fritzl's basement workshop, protected by an electronic code known only to Fritzl, which he entered using a remote control unit. In order to reach this door, five locking basement rooms had to be crossed. To get to the area where Elisabeth and her children were held, eight doors in total would need to be unlocked, of which two doors were additionally secured by electronic locking devices.
Key events

The sequence of key events in this case is as follows:
Date Key event
1977 Fritzl begins sexually abusing his 11-year-old daughter, Elisabeth.
29 August 1984 Fritzl lures Elisabeth, now 18 years old, into the basement and imprisons her.
November 1986 Elisabeth has a miscarriage in the 10th week of pregnancy.
1989 The first child, Kerstin, is born, and lives in the cellar until 2008.
1990 Stefan is born. He, too, stays in the cellar until 2008.
1992 Lisa is born. In May 1993, when she is nine months old, she is discovered outside the family home in a cardboard box, allegedly left there by Elisabeth, along with a note asking for the child to be cared for.
December 1994 The fourth child, ten-month-old Monika, is found in a stroller outside the entrance of the house. Shortly afterwards, a phone call is made to Rosemarie, apparently, from Elisabeth. The caller asks Rosemarie to take care of the child. However, it is assumed that Fritzl was able to use a recording of Elisabeth's voice to make the call. Rosemarie reported the incident to the police, expressing her astonishment that Elisabeth knew of their new and unlisted phone number.
May 1996 Elisabeth gives birth to twins. One dies after three days and Fritzl is alleged to have removed his body from the cellar and cremated it. The surviving twin, Alexander, is taken upstairs when he is 15 months old. He is "discovered" in circumstances similar to those of his two sisters.
December 2002 Felix is born. According to a statement by Fritzl, he kept Felix in the cellar, together with Elisabeth and her two eldest children, because his wife was not able to look after another child.
19 April 2008 Fritzl arranges for the critically ill 19-year-old Kerstin to be taken to a local hospital.
26 April 2008 During the evening, Fritzl releases Elisabeth from the cellar along with her sons Stefan and Felix, bringing them upstairs, informing his wife that Elisabeth had decided to come home after a 24-year absence. Later that evening, after an anonymous tipoff during a visit to the hospital, Fritzl and Elisabeth are taken into police custody where she reveals her decades-long imprisonment during questioning.
16 March 2009 Trial commences in St. Pölten, Austria.
18 March 2009 Fritzl changes his plea to guilty on all charges.