London, May 2 (ANI): Elizabeth Fritzl, who was held captive and regularly raped by her father for 24 years, was left alone in her dungeon for three days as she gave birth to her warped father's twins, it has emerged.

The now 42-year-old Elizabeth watched helplessly as one of the babies died from breathing difficulties in her dank underground prison home.

When evil dad Josef finally turned up he callously tossed the baby's body into a furnace.

Fritzl, 73, now faces a charge of first-degree murder over the death of the baby, his lawyer confirmed on May 1.

As the full extent of Fritzl's cruelty emerged, cops said Elisabeth was also left on her own to give birth to other children that resulted from rapes by her dad.

"We thought we were dealing with a monster, but this man is the Devil himself. We think he went off sex with Elisabeth when she was heavily pregnant and left her alone to have her babies underground," The Sun quoted a police source, as saying.

"All he cared about was satisfying his lust and keeping the terrible secret of his hidden family. The more we learn about him, the more his actions defy belief. He is morally sub-human," the source added.

As the case of Austrian incest dad deepens, the cops are suspicious that sex beast Josef Fritzl's wife knew more than she's letting on.

The suspicions clouded on the 67-year-old Rosemarie Fritzl, after it was claimed that she took food supplies to the cellar and may have received shopping ordered in Elisabeth's name.

Rosemarie's 73-year-old husband has insisted that she had no idea he had imprisoned Elisabeth beneath their home for more than two decades and fathered seven children by her in Amstetten, Austria.

She was said to have been told by engineer Fritzl he spent night after night in the cellar "drawing up blueprints for machines".

But a source close to the inquiry said that the questions over how much Rosemarie knew were mounting up.

"The questions over how much Rosemarie knew are mounting," The Sun quoted the source, as saying.

Investigators said it "defied logic" that she could not have been suspicious of deliveries of clothes and other goods addressed to "missing" Elisabeth, now 42.

It has also emerged that 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." (ANI)