Police probing the case of a man who held his daughter captive underground for 24 years are investigating whether he set up a mechanism to gas the inhabitants of the cellar dungeon where he kept a secret family imprisoned.
Federal Bureau of Investigations spokesman Helmut Greiner said Josef Fritzl, 73, had made the claim during initial police questioning, The Associated Press reported.
Police say Fritzl confessed to keeping his daughter Elisabeth in his basement for 24 years, repeatedly raping her and fathering her seven children -- six of whom survived, three living underground with their mother.
Fritzl is alleged to have threatened to gas his prisoners if they harmed him or tried to escape, AP says.
Meanwhile, Fritzl's sister-in-law revealed further details Thursday about the secret double life of the man whose exploits have stunned Austria. In an interview with the Oesterreich newspaper, she said Fritzl had often spent entire days and nights in the cellar.
"Every day at nine in the morning Joseph would go into the cellar. He said he was drawing engineering plans that he would sell to companies," the woman -- identified as Christine R, the sister of Fritzl's wife -- said.
"Often he would stay there all night" and his wife was not even allowed to bring him coffee, she added.
Austrian authorities said the imprisoned children, who for years had not seen the light of day, were slowly adapting to sunlight. Officials also debunked reports in a few British newspapers that some of the children could not walk or speak in sentences.
The story of the family's imprisonment began to unravel a week ago, when Elisabeth Fritzl's oldest daughter, Kerstin Fritzl, fell seriously ill with convulsions and was hospitalized.
The 19-year-old girl, who was locked in the basement along with her mother and two brothers, remains in an artificially-induced coma in an Amstetten clinic. She is suffering from a kidney ailment that worsened because she did not receive medial treatment sooner, authorities said.
"We cannot expect any dramatic changes in her condition in next few days," Dr. Albert Reiter said. "We will make every effort to help as best as we can, perhaps in a few weeks we hope some positive change for the better."
Fritzl had told his wife that Elisabeth, who is now 42, ran away from home aged 18, police say. The Fritzls adopted three of the children who Josef said were left on his doorstep as infants by his runaway daughter. Fritzl has confessed to incinerating the body of the infant that did not survive, according to police
Fritzl has yet to be charged, but he can be held by police for 14 days without formal charges while the investigation is under way. That amount of time can be extended by a judge.
A spokesman for the prosecution, Gerhard Sedlacek, said Fritzl had stopped talking to police since making his initial confession. Investigators intend to begin questioning him again next week, Sedlacek said.
Austrian police spokesman Franz Polzer said the investigation would likely last a couple of months. Police plan to interview at least 100 people who lived in the same apartment building as the Fritzls over the past 24 years.
"Investigation officials will go back as far as they can in order to discover what his behavior and actions were at that time ... but it is not very easy to reconstruct 24 years," Polzer said.
Polzer also said police were investigating suggestions that Fritzl may have been making plans to release his daughter. Investigator discovered a letter by Elisabeth, apparently written on Fritzl's instruction, in which she said she wanted to return home but "it's not possible yet," AP reported.
"He may have had plans to end the captivity at some point," Polzer said. "It just shows how perfectly he planned everything. It shows that he must have had a spark of humanity."
Authorities are also looking into media reports that Fritzl may have had a prior rape conviction in the 1960s, but have no information because criminal records are expunged after a certain number of years under Austrian law.
However, Austrian police spokesman Franz Polzer denied reports that authorities were looking into Fritzl's ties to the unsolved murder of a young woman more than 20 years ago.
During the time Fritzl owned a hotel and restaurant at a lake in Austria, a woman was found murdered at the other end of that lake, Polzer said.
Police are aware of the media reports and may investigate possible links in the future, but at this time, Polzer said, there is no investigation.
The investigation will also examine whether any accomplices were involved in the case after police revealed they had received an anonymous tip-off that Fritzl had imprisoned his daughter. But Polzer said there were "neither technical nor biological traces of any other person in cellar" other than Fritzl.
On Sunday, Elisabeth and her two sons, ages 18 and 5, met the three children who were raised by Josef and Rosemarie Fritzl, unaware that their mother and siblings were kept prisoners in the basement.
"It is astonishing how easy it worked, that the children came together and also it was astonishing how easy it happened that the grandmother and the mother came together," clinic director Berthold Kepplinger told reporters Tuesday.
Authorities are looking into the possibility of giving new identities to the Fritzl family. District Governor Hans Heinz Lenze said Tuesday that the Fritzl name had been "muddied" by the case.
On Wednesday, a German tabloid released video of Fritzl, laughing on a Thai beach and receiving a massage -- apparently as his daughter and three of their children remained locked up in his basement.
The newspaper Bild posted the video on its Web site, and said it was taken and provided to the newspaper by Fritzl's friend, a retiree from Munich.