London, May 2 : If keeping a secret family captive in his cellar for two decades was not enough, dungeon dad Josef Fritzl, threatened to gas them to death if they tried to overpower him and escape, the Austrian police reported.
The 73-year-old monster, allegedly told his prisoners that he had installed a device that could pump out a poisonous substance and they would be left entombed in their windowless dungeon.
Fritzl locked up his daughter Elisabeth, 42, in the cellar under his home in Amstetten for 24 years.
Three children he fathered by her - Kirsten, 19, Stefan, 18, and Felix, five - lived their whole lives underground until they were released earlier this week.
"We are trying to ascertain whether there really was a mechanism that would allow the gas to be pumped in or whether it was to intimidate them," the Telegraph quoted Austrian police spokesman Helmut Greiner said, as saying.
Fritzl would often sleep overnight with his captives in the cellar and hadn't had sex with his wife Rosemarie, 68, upstairs for 20 years.
Fritzl and Rosemarie had seven grown up children. Three other children he fathered with Elisabeth - Lisa, 16, Monika, 14, and Alex, 12 - lived upstairs in the house.
According to Rosemarie's sister Christine, 56, Fritzl treated his family like a "military dictator" and spent more time in the cellar than above ground.
"He would go down there at 9am and often spend the night there. He said he was developing blueprints for machines he was selling. No one was allowed down there. Rosie wasn't even allowed to bring him a cup of coffee," Christine said.
"He always put Rosie down and called her fat. He would say 'Fat women are below my standard'," she added.
Fritzl was also so vain about going bald that he had hair transplants.
Christine said: "He was a tyrant and despot. I have always hated him. He treated his children like a drill instructor. It was like they were in the army."