An elderly Austrian, portrayed by media as a "monster", confessed on Monday to imprisoning his daughter in a cellar for 24 years and fathering her seven children, prosecutors said.

Josef Fritzl, 73, "has admitted building the dungeon and to holding his daughter and three children there," prosecution spokesman Gerhard Sedlacek said.

Fritzl also admitted incest "but insisted there was no force involved," said Sedlacek. One of the children died at an early age.

Fritzl was scheduled to be brought before an investigating magistrate on Monday evening and face several more days of questioning over the case which has shocked Austria.

Fourteen police meanwhile scoured the three cramped underground rooms in the family house in Amstetten, eastern Austria, where Elisabeth Fritzl and her children were held prisoner.

She has alleged she was drugged by her father in August 1984 and had been his prisoner ever since. All seven children were born in the "dungeon".

The six surviving children are three boys and three girls aged between five and 20.

The rooms, measuring "50-60 square metres in all" and with a ceiling just 1.70 metres (5.5 feet) high were "furnished like a flat", Sedlacek said.

Lower Austria police chief, Franz Polzer, said there was "a wide range of questions that still need answering" such as how Fritzl supplied the woman and children with food, how the babies were born and cared for in such cramped conditions, and how he could have incarcerated his victims for so long without his wife knowing.

It is the latest in a series of horror abuse cases to have stunned Austrians and newspapers asked how authorities could again have failed to detect the woman.

The case came to light when one of the children, now 19, was admitted to hospital in critical condition.

Doctors looking for background information stepped up efforts to find the mother. The whole horrific story came to light when Fritzl allowed them to establish contact with his daughter.

The Oesterreich tabloid featured a six-page special report on what it termed as "the worst crime of all time." "How can this happen here?" asked Die Presse .

"Amstetten is in a state of shock," wrote mayor Herbert Katzengruber on the town's website. "Our thoughts and feelings are with the victims."

DNA tests are being carried out to establish if Fritzl is the father of the six surviving children. The seventh child, a twin, is believed to have died shortly after birth and the body subsequently burned, police said.

The Kronen-Zeitung tabloid portrayed Fritzl as a keen fisherman, popular among neighbours and locals, but a "monster, a brutal tyrant" in the cellar of his own home.

He legally adopted two of the boys and one girl, allegedly telling his wife, Rosemarie, and local authorities that three babies had been left by Elisabeth on their doorstep, in different years.

Each delivery was accompanied by a letter purportedly signed by Elisabeth Fritzl saying she could not support the child because she already had others to care for.

The trio went to school as normal, seemingly unaware that their mother and three other siblings (a girl of 19, and boys at 18 and five) were trapped underground.

Neither neighbours nor social services appear to have had any inkling, either.

"They had a swimming pool in the garden, we would hear them laughing the three of them," said one neighbour.

Another backed up the story of the babies on the doorstep, adding: "(Rosemarie) always looked after the kids so well, taking them to school. We said 'it's incredible what she manages to do at her age'."

Elisabeth Fritzl told investigators her mother knew nothing about the sexual abuse she had endured since the age of 11, some seven years before she was locked away.

Austria's most notable prior case was that of Natascha Kampusch, locked up by a man in the basement of a house for eight years before she escaped.

Kampusch was 10 when Wolfgang Priklopil abducted her on her way to school in 1998. The 44-year-old kidnapper killed himself hours after she fled, throwing himself under a train.

Three young girls were also locked up for seven years by their mentally ill mother near Linz.

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