Josef Fritzl may have had a Second sex slave in his cellar dungeon - granddaughter Kerstin.
Police fear the monster targeted his eldest child by caged daughter Elisabeth after losing interest in her grey-haired mother, reported The Sun.
But they have been unable to quiz the 19-year-old because she is in a coma, fighting for her life in hospital with organ failure.
It is believed Fritzl, 73, was fed up with 42-year-old Elisabeth - whose looks had been ravaged by 24 dark and brutal years locked up under his home in Amstetten, Austria.