DNA tests have confirmed that a 73-year-old Austrian electrical engineer is the father of his daughter's six surviving children.

In the case dubbed "The House of Horrors", Josef Fritzl has appeared before a judge in St Poelten, the provincial capital of Lower Austria, accused of keeping Elisabeth Fritzl, 42, in a cellar for 24 years and fathering seven children by her.

Franz Polzer, head of the criminal investigation unit in Lower Austria, said: "The result... shows that the six children, which the unfortunate Elisabeth Fritzl gave birth to in the basement, have all been undoubtedly fathered by her own father, the now 73-year-old Joseph Fritzl."

Fritzl is also being investigated over the death of the seventh child, a twin who died shortly after birth, and could face a charge of killing the baby through neglect. He has admitted to burning its body in a furnace used to heat the building.

Local chief public prosecutor Peter Ficenc said: "Josef F is being investigated for murder by failing to render assistance." He added that an inquiry is also being carried out over rape, incest and coercion.

Elisabeth - abused from the age of 11 and lured into the cellar at the age of 18 - claims she was drugged and handcuffed before being imprisoned. Police said a letter apparently written by her surfaced a month after her disappearance in 1984, asking her parents not to search for her.

He claimed she had joined a sect and that she had left three children - two girls and one boy - on the doorstep of their home. They were adopted and raised by Fritzl and his wife Rosemarie.

Elisabeth lived with three other children - a girl now aged 19 and two boys aged 18 and five - in the 60-square metre (645 sq ft) basement in Fritzl's nondescript two-storey home, which officials said was no more than 1.7 metres (5 ft 6 in) high and contained a padded cell.

Fritzl, whom police described as "dynamic, bossy and authoritarian", had hidden the entrance to the cellar behind shelves, and only he knew the code for the concrete door.

The case was exposed when Elisabeth's 19-year-old daughter became seriously ill and was taken to hospital with severe cramp caused by lack of oxygen. Doctors appealed for her mother to come forward to give details of her medical history.

Fritzl brought Elisabeth and her remaining two children out of the cellar, telling his wife their "missing" daughter had chosen to return home, police said. Mrs Fritzl did not know what happened to her daughter when she disappeared, police have said.

He faces up to 15 years in prison if charged, tried and convicted on rape charges, the most grave of his alleged offences under Austrian law, officials said.

Meanwhile, investigators have been combing through the cells, removing boxes of evidence from the building, which is home to several other families.

Media commentators are now asking how the house, situated in a busy street with shops in the small industrial town of Amstetten, passed unnoticed for so long, particularly as Fritzl built extensions to the cellar.

Petra Stuiber wrote in Austria's Der Standard newspaper that what she termed a rich self-satisfied society needed to examine why the crimes were allowed to occur. "How is it possible that nobody heard or saw anything? How can it be that nobody asked questions?".

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