Investigators plan to use sonar technology to probe the yard of an Austrian man who held his daughter captive for 24 years to ensure that no more underground dungeons exist on the property, police said Friday.

Leopold Etz, chief of homicide investigations for Lower Austria province, said investigators are also questioning more than 100 people who lived in Josef Fritzl's house.

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

Those people lived in the building over the years that Fritzl held his daughter Elisabeth prisoner in a secret dungeon, fathering seven of her children.

Others who have come forward saying they knew the 73-year-old Fritzl are also being questioned.

"We're casting a wide net," Etz said. "It's a lot of work."

He said officers were combing the entire property, photographing, filming and mapping it. They also plan to use sonar to determine if there are any more secret underground spaces on the property, Etz said.

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 a hospital suffering from an illness linked to an unidentified infection.

Baffled doctors appealed on TV for Kerstin's mother to come forward because they needed information about the girl's medical history. Fritzl accompanied Elisabeth to the hospital on April 26, and her story came to light.

Klaus Schwertner, a spokesman for the family's medical issues, said Kerstin remains in critical condition "but has stabilized somewhat in recent days."

He declined to confirm a report that she is suffering from multiple organ failure, but said she remains in an induced coma at the Landeskrankenhaus Amstetten.

Etz also said authorities were trying to verify whether a mechanism existed to pump gas into the dingy, windowless rooms where Elisabeth lived with Kerstin and two of her sons, as Fritzl had claimed during initial police questioning.

Authorities have said the house had an official gas line, but for now they believe Fritzl's threat was nothing more than an attempt to keep his captives from trying to escape.

Police Col. Franz Polzer, who heads the criminal investigation, said investigators have determined that the entrance to the dungeon was protected by a reinforced double steel door that opened and closed using a remote control.

Investigators working in the underground rooms had to take frequent breaks due to a lack of oxygen.

"We are trying to think of some way to improve the air circulation," Polzer said.

Former tenants of the house have said that Fritzl told all residents of the apartment house that the basement was off limits and that they were not allowed to take photos in the area. Anyone who broke that verbal agreement was threatened with eviction.

A son and two daughters of Elisabeth by Fritzl were removed from the cellar by him when they were babies. He and his wife, Rosemarie — who was told that Elisabeth had abandoned the children and was in a sect — officially adopted one and were granted custody over the others. A seventh child died as an infant and Fritzl has confessed to burning its body in an incinerator.

Josef Schloegl, head of the Amstetten district court that approved the Fritzl's adoption of the first child in 1994 and awarded them custody of a second several years later, said the couple had no criminal records and appeared entirely normal.

"The grandmother cared for the children in an exemplary manner," Schloegl said.

The Fritzls never formally applied for legal custody of the third child, but were allowed to keep it, he said, noting all three children were "inconspicuous," "popular" and well taken care of.

Fritzl 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 crime punishable by up to 20 years in prison.