Josef Fritzl, the Austrian man who held his daughter and children captive in a windowless cellar under his house, had told his victims that they would be gassed to death if anything happened to him, investigators say.

Police experts were today examining Mr Fritzl's house in the small town of Amstetten to investigate his claim that that gas would be pumped into the dank 60-square-metre cellar, said Helmut Greiner, a spokesman for Austria's Federal Bureau of Investigations

“It may have just been an empty threat to intimidate the captive woman and her children into not trying to overpower him,” Mr Greiner said.

Mr Fritzl, a 73-year-old retired electrical engineer, has already admitted that he imprisoned his daughter Elisabeth, now 42, in the cellar of the family home for 24 years, fathering seven children by her. Three of those children grew up with her in the windowless dungeon.

But in a separate development, the officer leading the investigation, Colonel Franz Polzer, said that Mr Fritzl had forced Elisabeth to write a letter last year which indicated that he may have been planning to release her from the cellar.

Colonel Pozler said that DNA testings proves that Elisabeth had written the letter to her family - who had been told by Josef that she had fled to a cult. In it, she wrote that she wanted to come home," but “it’s not possible yet".

“He may have had plans to end the captivity at some point,” the officer said.

Mr Greiner said that the police technicians were trying to work out whether Mr Fritzl had indeed installed some kind of device to pump poison gas into the cellar.

Experts were also examining another claim by Mr Fritzl that the heavy concrete-reinforced door to the underground rooms where he imprisoned and sexually abused his daughter, would have sprung open automatically were he absent for an inordinate length of time.

“That also may be simply an empty claim,” Mr Greiner said. Either claim could have possible implications for sentencing when the case comes to trial, because, depending on their veracity, the victims would have either have been able to escaped or could have perished, he noted.

In the first interview by a member of Mr Fritzl's extended family, his sister-in-law has revealed that the engineer often spent the whole night with his daughter and their children in the dungeon under the house.

The woman, named only as Christine R, 56, said that Mr Fritzl made daily visits to his cellar on the pretext that he had work to do and would tell other family members, including his wife Rosemarie, not to disturb him.

“Sepp [Josef’s nickname] would go to the cellar every day at nine in the morning and he would often even spend the night there. Allegedly, to draw blueprints for the machines he was selling," she told the Oesterreich newspaper.

"Rosi was not even allowed to bring him a cup of coffee."

Police say that Mr Fritzl began abusing his daughter when she was aged just 11, but held her captive from the age of 18. When she became pregnant from the abuse, he began to expand the underground dungeon from initially one room to three.

No one was aware of the existence of the dungeon, not even fire inspectors who routinely checked a heating boiler in the cellar in 1999.

The entrance to the dungeon was reachable only via a labyrinth of different underground rooms. The door itself, about one metre high and 60 centimetres wide, was reinforced with concrete and had an electronic lock that could only be opened with a remote control.

Mr Fritzl was a retired electrical engineer and police believe he had the expertise to build such a door and install it by himself.

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