Investigators are trying to determine whether an Austrian man who held his daughter and three of the children he fathered with her in a basement cell for 24 years had constructed a way to gas the chamber if they ever tried to escape.

Josef Fritzl allegedly told police during questioning that he repeatedly threatened to gas his captives, who were kept just below the apparently normal living quarters shared by Fritzl and his wife, Rosemarie.

Fritzl, 73, has confessed to locking his daughter Elisabeth in the cell when she was 18 and repeatedly raping her, telling his wife that the girl had run away from home to join a religious cult.

Elisabeth — who is now 42 and until recently hadn't seen the light of day since her capture — gave birth to seven of her father's children in the windowless cellar. One child apparently died and was later thrown into an incinerator by Fritzl.

A spark of humanity?

Austrian authorities are also investigating whether Fritzl coerced his daughter into writing a letter last year that suggested he was considering her release. The letter, sent to her family, said Elisabeth wanted to return home but that "it's not possible yet." DNA tests prove she wrote the letter.

"He may have had plans to end the captivity at some point," Police Col. Franz Polzer told the Associated Press. "It shows that he must have had a spark of humanity."

The case came to light last week when Elisabeth's 19-year-old daughter, Kerstin, became ill and was taken to hospital by Fritzl. Suspicious of the man's behaviour, authorities compared Fritzl's DNA to a sample taken from Kerstin, and made a televised appeal for the mother to come forward with information on the girl's medical history.

After having persuaded her father to let her meet doctors in her first trip outside the basement in 24 years, Elisabeth and her father were detained on April 26, providing an opportunity for the woman to finally reveal her ordeal to authorities.

Kerstin and her two siblings, aged 18 and 5, had never been outside the tiny cellar since they were born there. Doctors said the young woman is in life-threatening condition due to an illness caused by an unidentified infection, and remains in hospital in an induced coma and on a respirator. She is also undergoing dialysis for a lack of oxygen, a hospital spokesman said.

Elisabeth and the other two are receiving treatment in a psychiatric facility.

Elisabeth's three other children were raised by Fritzl and his wife, who was told by her husband that Elisabeth had sent the children home from the religious cult to be raised by their grandparents.

Wife unaware of husband's actions: psychologists

Psychologists who observed the emotional reunion between Elisabeth and her mother this week concluded that Rosemarie was unaware her daughter was being held in captivity.

Meanwhile, a man who stayed at the house in Amstetten, west of Vienna, where Elisabeth and her children were held has told the BBC he saw a man described by Fritzl as a plumber visit the basement cell.

Alfred Dubanovsky, who lived in the house for 12 years, said he was told never to enter the cellar or he would be evicted. He said he often heard sounds coming from the subterranean chamber but was told by Fritzl that it was just the heating system making noise.

Fritzl's sister-in-law, identified by the Austrian press as Christine R., told reporters that Fritzl would visit the cellar at 9 a.m. each morning "apparently to draw plans for machines, which he sold to firms."

"Often he even stayed down there for the night," she told the Oesterreich newspaper. "Rosi [his wife] wasn't allowed to bring him a coffee."

Christine described her brother-in-law as a "despot, I hated him," saying he "always belittled" his wife.

Despite his earlier confessions, Fritzl has refused to answer further questions from police.

He faces up to 15 years in prison if convicted on rape charges, and 20 years if prosecutors decide to charge him with "murder through failure to act" in connection with his young child's death and incineration.

With files from the Associated Press