AMSTETTEN, Austria (CNN)
-- DNA tests have confirmed that Josef Fritzl fathered six children with his daughter, whom he kept imprisoned under his home for 24 years, police in Austria said Tuesday.
Authorities have said that a seventh child, also apparently fathered by Fritzl, died shortly after birth.
A police spokesman also said authorities believed it was "improbable" that Fritzl was hiding other people in another location.
The 73-year-old appeared in court Tuesday as Austrians struggled to come to terms with its second horrific abduction case in two years.
The retired electrical engineer faces a possible 15 years in prison if charged and convicted of rape.
In court, he appeared calm and showed no emotion, according to an Austrian reporter present. Fritzl spoke, saying that he wanted to redeem himself, said Sabrine Arndt, of Pro7/Sat1 TV station.
The female judge presiding said he would be kept in custody, the reporter said, but gave no indication for how long.
Fritzl was on Monday moved from the town of Amstetten -- where he kept his now 42-year-old daughter Elisabeth and three of her children in the cellar of his house -- to the courthouse in nearby St. Poelten, the provincial capital of Lower Austria.
The central European country's newspapers were filled with details of the case, which has shocked the nation.
"Horror father breaks silence," "Hiding a double life without wife knowing" and "Soundproof dungeon behind 300kg steel door" were headlines in Die Kronen Zeitung.
It also ran a story questioning how people could survive deprived of sunlight for so long.
Die Presse went with "The man who deceived the world" and also accused authorities of allowing the atrocities to happen.
Thomas Birgfellner, a reporter with Austrian broadcaster ORF, said there was a strong belief that Fritzl -- who installed an electronic security door in the cellar -- must have had help from other people.
"Everyone has said he could not do it alone. He could not install it alone, and now they have to investigate if there were some other people who assisted him," Birgfellner told CNN.
CNN's Phil Black reported that Austrian police were trying to deflect comparisons with the case of Natascha Kampusch, who 18 months ago escaped from a basement cell near Vienna in which she had been held since she was kidnapped as a 10-year old on her way to school.
"There are similarities on the surface but police say this case is more extreme, and they do not believe that there is anything darker or more rotten here in Austria than in any other country," Black said.
He said the police were now trying to work out how Fritzl was able to deceive the authorities and his neighbors for so long.
The three children held in the cellar were still in hospital on Tuesday having treatment following their horrific ordeal.
The eldest of the children, 19-year-old Kerstin Fritzl, was in an induced coma.
It was her serious condition that led to the unraveling of the case at the weekend when her alleged father Josef -- who she thought was her grandfather -- was forced to take her to the hospital in Amstetten, west of Vienna.
Elisabeth said one of her seven children died at an early age due to inadequate care, while three were adopted by Josef and his wife Rosemarie and lived upstairs in the house.
Three were kept with her in the cramped cellar: Kerstin and sons Stefan, 18, and Felix, 5.
The boys are said to be doing "surprisingly well and in good health" in the circumstances but are still undergoing medical treatment, Black said.
The breakthrough in the case began a week ago when Kerstin Fritzl fell seriously ill with convulsions.
Her mother Elisabeth begged Josef to take her to a hospital, which he did.
Josef told his wife that their missing daughter had dropped off ailing Kerstin on the doorstep with a note asking that they get her medical care.
Josef took her to the town's clinic with the note, but doctors needed more information to determine why the young woman was unconscious and having violent convulsions.
So they contacted police, who asked the local media to report on Kerstin's situation in an effort to find the missing mother.
Elisabeth and her two sons saw the reports on the television provided to them in their 100-square-foot living quarters, police spokesman Franz Polzer said, and "they desperately pleaded with their father so they could be taken (to the hospital)... (and) do something for the 19-year-old."
Josef Fritzl agreed, and took all three of the remaining captives out of the basement, explaining to Rosemarie and the rest of the family that Elisabeth had reappeared with her two children after an absence of 24 years.
He took them to the hospital and, at some point, authorities there realized something was wrong. Police picked up both Josef and Elisabeth on Saturday near the hospital and brought them into the station for questioning.
Josef would not talk to police but Elisabeth began to tell her story once she was convinced she would never have to see her father again, and that her children would be safe.
"The young woman saw the window for her freedom and she entrusted herself to the criminal officer and began to talk about the 24 years in captivity," Polzer said.
She told police her father began sexually abusing her at age 11. On August 8, 1984 -- weeks before she was reported missing -- her father enticed her into the basement, where he drugged her, put her in handcuffs, and locked her in a room, she told police.
For the next 24 years, she was constantly raped by her father, resulting in seven children, six of whom survived, she said, according to the police statement.
She told police she gave birth to twins in 1996, but one of the babies died a few days later as a result of neglect, and Josef Fritzl removed the infant's body and burned it.
Elisabeth told police that only her father supplied her and her children with food and clothing, and that she did not think her mother, Rosemarie, knew anything about their situation.
Police continued to question Josef Fritzl and he led police to the underground cellar on Sunday. A day later, he confessed to raping his daughter, keeping her and their children in captivity, and burning the body of the dead infant in an oven in the house.