The sister-in-law of Josef Fritzl, the man who imprisoned and raped his daughter, shocking Austria, declared that Fritzl had also been convicted for rape in 1967, serving 18 months in prison, the Associated Press reports.

The woman also insists that Rosemarie Fritzl had no idea of her husband’s double life and never suspected him of being involved in the 24-year disappearance of their daughter.

Rosemarie’s sister says that the daughter ran away when she was 17, about six months before police say she was imprisoned by her father in the cellar.

Austrian police investigating the case were shocked by the conditions in which Fritzl’s “second family” lived. They said the air in the inner chambers was so stifling they had to drill holes so they could breathe during the investigation.

They have discovered several cells that were used for solitary confinement for his daughter Elisabeth and her three children, who had been fathered by Fritzl. It seems Fritzl was using the cells as a method of punishment.

Police chief Franz Polzer told the Associated Press that officers could "work there only one hour and during this hour they try, one team after the other, to gather everything available in this living space".

Fritzl reportedly told his victims they would be gassed in case anything happened to him.

Technicians are currently trying to establish whether that was possible or it was only a threat.

As the cellar’s structure is complex, it seems the investigations could last several weeks.

The terrible story was made public when Elisabeth’s eldest daughter Kerstin, 19, was taken to the hospital, suffering from multiple organ failure.

Kerstin had lived her entire life in the dark dungeon and is now in a coma. Medical experts said she had no chance to recover. Kerstin and her two other brothers, aged 18 and 5, had never in their life seen the daylight.

Elisabeth had three more children with her father, but they were raised upstairs by Fritzl and his wife, who claims she had no idea about who they were, saying they mysteriously turned up at their doorstop.

Fritzl faces charges of incest, abduction and imprisonment and could serve 15 years in jail if convicted.