Authorities are bracing for a "tragic ending" as they prepare to dig in an "area of interest" they located in the backyard of a Northern California couple charged in the kidnapping of a little girl — and suspected in the disappearances of two others.

Hayward, Calif. police used bone-sniffing dogs and a high-tech sonar device to zero in on a piece of property owned by Phillip and Nancy Garrido, who authorities say kept a captive girl locked away in a soundproof backyard compound for 18 years.

A human remains detection dog — who can pick up the scent of bones hundreds of years old buried deep under ground — was brought to the backyard and alerted in the same area that cadaver dogs had "indicated" remains during a search last week, said Hayward Police Lt. Chris Orrey.

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The spot is also the site of an underground anomaly located last week by a ground-penetrating radar device (GPR), Orrey said at a press conference Monday afternoon.

Authorities have conducted an exhaustive search of the Garrido's house and property, hoping to find evidence related to the disappearances of other young girls during the 1980s, but have so far not found anything noteworthy.

"We didn't find anything of significance that would help us with the investigation so far," Orrey told reporters. "Now if we find anything it's going to be a tragic ending."

The Garridos are charged in the 1991 kidnapping of Jaycee Dugard, who was snatched outside her South Lake Tahoe home when she was 11 years old. Prosecutors say the Garridos held her captive for 18 years as Dugard was raped repeatedly by Phillip, who fathered two children by her. The couple has pleaded not guilty.

Police officials and the FBI are seeking any evidence on the property that may link the Garridos to the 1988 abduction of Michaela Garecht, then 9, outside of a market. Authorities are also looking for clues that may tie the couple to the 1989 disappearance of Ilene Misheloff, who was then 13.

Orrey said that police have finished their investigation inside the Garridos' home, and are preparing to end their work in the backyard if excavations planned for Monday do not turn up any evidence.

"If we don't find anything of significance we hope to wrap up the operation (Tuesday)," she said.

On Wednesday, police recovered a bone fragment on the Garrido property 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, and have collected personal items that they wouldn't identify that they say belong to Dugard.