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The cellar in Austria where Josef Fritzl kept his daughter captive for 24 years, and fathered seven children with her, is being filled with concrete.

Preparation work began in Amstetten on Thursday and will last about two weeks.

The man appointed to liquidate Fritzl's estate told the Austrian news agency, APA, that the step was being taken to ensure that no-one could ever enter the cellar again.

Fritzl, 78, was jailed for life in March 2009.

He was convicted of murdering one of his children through neglect, as well as rape, incest, and enslaving his daughter.

The authorities were said to be concerned that the scene of the crimes could become a gruesome tourist attraction.

The cellar had already been sealed from the inside.

Small holes are now being drilled, through which tubes will be fed to fill the underground rooms with concrete.

In 2011, authorities in the town of Amstetten announced plans to demolish the house, but they were not carried out.

Instead, the building will now be sold.

The estate liquidator, Walter Anzboeck, said there were potential buyers, including builders.

"But the aim was always - and we said this before - to sell the house only if the cellar was not accessible any more", he said.

Fritzl's daughter and her children have taken on new identities and now live at an undisclosed location in Austria.