A letter Josef Fritzl forced his captive daughter to write late last year indicates he may have been planning to release her from the cellar where she was held captive for more than two decades, police said Wednesday.

The revelation came as authorities released another chilling detail of the "house of horrors:" The man who fathered seven children with his daughter repeatedly warned his dungeon prisoners that they would be gassed if they harmed him and tried to escape.

Police Col. Franz Polzer said Elisabeth Fritzl wrote to her family, who believed she had fled to a cult, that she wants to come home but "it's not possible yet."

DNA testing on the letter proved that 42-year-old Elisabeth had written the letter, but Polzer said she was forced by her father to do so.

"He may have had plans to end the captivity at some point," Polzer told The Associated Press on Thursday. "It just shows how perfectly he planned everything."

• Click here to see photos of Josef Fritzl and the "House of Horrors."

Fritzl's elaborate crime came to the attention of authorities on April 19 when one of Elisabeth's daughters, 19-year-old Kerstin, was admitted to hospital suffering from an illness linked to an unidentified infection.

Baffled doctors then appealed on TV for Kerstin's mother to come forward because they needed information from her about her daughter's medical history. Fritzl then allowed Elisabeth to go to hospital, and her story came to light.

"It shows that he must have had a spark of humanity," Polzer said.

Meanwhile, Federal Bureau of Investigations spokesman Helmut Greiner said investigators were checking to verify whether Fritzl had indeed set up a mechanism that could send gas into the dingy, windowless enclosure as he had claimed during initial police questioning.

Experts are also checking another Fritzl claim that the reinforced door leading to the enclosure had a timer that enabled it to be easily opened if he was gone for an unusually long period of time, said Greiner, spokesman for the Federal Bureau of Investigations.

Days before Elisabeth told police of her 24 years in captivity, law enforcement officials had become suspicious and started investigating Fritzl, according to a police statement.

It said police decided to compare DNA samples of the hospitalized Kerstin and of Fritzl, along with family members living with him in late April, about 10 days before revealing the ordeal to the public. Fritzl and Elisabeth were detained April 26 near the hospital where Kerstin was being treated and Elisabeth then told her story to interrogators.

The letter is one part of evidence that authorities are using to piece together Josef Fritzl's double life as reputable citizen and "horror father" who allegedly held his own daughter and three of their children captive.

One of the children she bore him died as an infant, and Fritzl has confessed to tossing the corpse into a furnace. He managed to smuggle the other three out to be raised by him and his wife, who was led to believe Elisabeth had given them up.

Police in Upper Austria are also examining whether Fritzl was also responsible for an unsolved murder in the nearby lakeside village of Mondsee two decades ago, where his wife owned an inn and camping ground.

The bound body of Martina Posch was found on a shore of the Upper Austrian lake of Mondsee on Nov. 12, 1986. The wife of the incest suspect owned part of an inn and camping ground on the other side of the lake at that time and police are investigating a possible link because Fritzl could have been in the area at the "time and place" of the killing.

"We have found no sign" of a concrete link up to now, Alois Lissl, police chief of Upper Austria said.

Compiling a complete profile of Fritzl has been difficult because he is refusing to undergo more questioning, police say.

The other dungeon children appeared to be doing relatively well. Berthold Kepplinger, director of the clinic where they are being treated, said they were "talking a lot" with each other.

"The family is doing well under the circumstances," Kepplinger said, considering they had to get accustomed to everyday conditions for others, such as daylight.

Elisabeth and her mother, who cared for the three children father Josef brought into his own apartment, were also "getting along very well," said Kepplinger. He said staff was trying to create the conditions for "the best possible start into a new life."

Leopold Etz, chief of homicide investigations for Lower Austria province, said authorities were confident that Fritzl acted alone.

"I think we can rule out accomplices," Etz told the AP.

He said DNA tests confirmed that no other man entered the soundproof cellar rooms Fritzl made into a prison below his home. On Tuesday, tests confirmed Fritzl as the biological father of his daughter's six surviving children.

Authorities say there was no evidence the suspect's wife, Rosemarie, knew what was going on or was involved.

Elisabeth "never said that her mother was in the cellar," Etz said.

Fritzl and his wife were granted custody over three of Elisabeth's children after the father concocted the cult story. He forged other letters and even made a fake phone call to his wife to convince her of Elisabeth's flight, Polzer said.

The father faces up to 15 years in prison if convicted on rape charges, the most grave of his alleged offenses. However, prosecutors said Tuesday they were investigating whether he can be charged with "murder through failure to act" in connection with the infant's death. That is punishable by up to 20 years in prison.

Investigators say a man who imprisoned his daughter and some of the children he fathered with her in a dungeon for more than two decades warned his victims they would be gassed to death if they tried to overpower him.

Federal Bureau of Investigations spokesman Helmut Greiner says Josef Fritzl repeatedly told his victims they would not survive if they harmed him in the cellar because gas would flow into the dingy, windowless area.

Greiner says federal investigators are checking to verify whether Fritzl had set up such a mechanism. Fritzl made the claim during initial police questioning.

Experts are also checking another Fritzl claim that the reinforced door leading to the enclosure had a timer that enabled it to be easily opened if he was gone for an unusually long period of time.