VIENNA -- Austrian Josef Fritzel would spend whole nights in the cellar of his home where he locked-up and raped his daughter Elisabeth for 24 years, his sister-in-law said in an interview published Thursday.

"He would go down into the cellar every morning at seven, supposedly to develop plans for machines that would sell to businesses," said the sister-in-law, identified only as Christine R. in the Oesterreich newspaper.

"Often, he would spend whole nights down there," added the 56-year-old younger sister of Fritzel's wife Rosemarie. "'Rosi' wasn't even allowed to bring him a coffee."

Police in Austria say Fritzel -- who held his daughter prisoner for 24 years and fathered seven children with her -- had no accomplices and planned his crime too meticulously to have been caught any earlier.

Christine R. recalled Fritzel's dark past, saying: "I was 16 years old when he was jailed for rape. I found that offence repugnant, given how he had had four children with my sister."

She described her older sister -- who police believe had no idea what evil her husband was up to downstairs at the family home in Amstetten -- as "dominated and constantly belittled in public" by Fritzl.