Alameda County Sheriff's Department spokesman Sgt. J.D. Nelson said two dogs "indicated" on a site in Phillip and Nancy Garrido's backyard. But he cautioned that the area is known to have buried remains from Native Americans and animals.
Investigators plan to pull up several concrete slabs Friday that seem randomly spaced on the property, reports CBS News Correspondent John Blackstone, who adds that probers want to look behind the walls and under the floors. If necessary, says Blackstone, they may tear down the entire house to learn its secrets.
The Garridos are charged with the 1991 kidnapping of Jaycee Dugard, who was snatched outside her South Lake Tahoe, Calif. home when she was 11. Prosecutors say the Garridos held her captive in their backyard for 18 years. The couple has pleaded not guilty.
Since the Garridos' Aug. 26 arrest, their Antioch home has become a focal point of investigators reviewing outstanding kidnapping cases in the San Francisco Bay area. Their lawyers are not discussing the property searches.
More on the Jaycee Dugard investigation on CBSNews.com
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Jaycee's Terror as Her Ordeal Began
Authorities from Hayward are seeking any evidence on the property that may link the Garridos to the 1988 abduction of Michaela Garecht outside a Hayward market. Dublin police are looking for clues that may tie the couple to the 1989 disappearance of Ilene Misheloff.
Investigators planned to use high-tech, ground-penetrating radar equipment to scan the site on Friday. They would start digging if the radar gives more specific information about what is underground, Nelson said.
Nelson would not say where in the yard the dogs picked up a scent and warned of false positives.
Cadaver dogs are trained to sniff for the scent of a decomposing body and can catch smells of up to two or three feet, Nelson said.
"They picked up a scent that may or may not be a sign of some remains," Nelson said. "The first dog was very tentative on its indication. The second dog was more direct and indicating directly."
Police say they also found numerous documents written by Phillip Garrido, along with personal items they wouldn't identify belonging to Jaycee.
Authorities have also released the first photos from inside the Garrido home. Blackstone says they "gave another glimpse into life there -- "a mess" that "could show a household in disarray -- or the way a place looks after a police search. CBS News spoke with a private detective who was in the house before the Garridos were arrested who says it wasn't in disarray.
On Wednesday, police and several more in a next door neighbor's yard that also was being searched by investigators because Phillip Garrido had access to it.
Tests are being done to determine if the bones are human or animal, officials said.
Investigators continued clearing trash and brush from the backyard that was outfitted with tents, sheds, an above ground pool and showers.