After investigators found additional bone fragments on Jaycee Dugard's accused kidnappers' property, cadaver dogs reacted at a spot on the property's soil.
"The first dog, by their description, was very tentative on his indication," said Sgt. J.D. Nelson of the Alameda County Sheriff's Department. "The second dog was more direct and indicating very directly."
Police cautioned that under some circumstances cadaver dogs will give a false positive. On Friday, authorities will begin using archeological dogs -- dogs that are able to detect older bones.
Additionally, it is not known whether the new bone fragments found on Garrido's property are human or animal bones.
Searchers likely will use ground penetrating radar and a magnetomer, technology that can read down 20 feet into the ground, before deciding to excavate on the spot where the dogs indicated.
"It's an open ground area," said Lt. Christine Orrey of the Hayward, Calif., Police Department. "We're going to run the X-ray equipment that I talked about over it ... to get a subterranean look at things, and we will eventually be digging in that location."
Investigators have been seen ripping down the sheds and tents on Garrido's property, and also crawling on their hands and knees, patting the ground and sifting through soil. Garrido and his wife, Nancy, allegedly kept Dugard prisoner in their backyard for 18 years, and officials have said he is the father of her two children.
While the search could go on for days, authorities said so far they've found nothing that breaks open the cases of 9-year-old Michaela Garecht and 13-year-old Ilene Misheloff, who were kidnapped in 1988 and 1989.
Police from Hayward and Dublin, Calif., where the girls disappeared, said this week they would be going through the Garridos' property, looking for evidence based on similarities between their abductions and that of Jaycee Dugard in 1991.
Michaela was snatched on a trip to a Hayward grocery store in broad daylight, and Ilene vanished on her way home from school.
An earlier search by police on the Dugard case, also of the Garridos' backyard and an adjacent property, turned up a bone fragment that was later determined to likely be human.
Investigators are planning to rip into the sheetrock at the Garridos' home, and are considering the possibility of razing the entire structure.
The lead official of the Hayward Police Department declined an interview today. Messages left with the Dublin Police Department were not immediately returned.
The two police departments are receiving assistance from teams of trained professionals, including the FBI.
Bob Lowery, executive director for the National Center for Missing and Exploited Children's missing children's division, said the composition of a backyard can still yield clues even 20 years later.
Digging for Clues: Garrido Property Searched for Clues in Disappearances
While Lowery is not involved in the physical search in Antioch, he has participated in similar evidence-gathering missions as commander of a violent crimes task force in St. Louis.
The most effective way to determine whether anything has been buried in the ground, he said, is to find places where the soil compaction may differ from the rest of the ground.
The crews that have been seen scouring the ground by hand, he said, are likely looking for areas where the soil is looser or softer. Different soil compactions, he said, also give off varying amounts of heat, so equipment to scan the ground's temperature can also be used to determine whether the earth has been disturbed.
And given the nature of the scene -- a backyard setting in a heavily residential area -- anything that's been buried on the property probably isn't very deep, and could be picked up by ground-penetrating radar.
Michaela's mother, Shannon Murch, told "Good Morning America" this week that she hoped the search would provide some answers as to what happened to her daughter more than 20 years ago.
"When Jaycee was found, my first thought was 'Please, God, let Michaela be with her,'" she said.
In 1988, Michaela was grabbed and pulled into a car -- while a young friend watched helplessly -- after the two had been given permission to take their scooters to a nearby grocery store. It was the first time she had been allowed to take the trip without an adult.
Murch said the friend called after seeing news reports on the Jaycee Dugard case to tell her the gray sedan impounded from the Garridos' property -- the same car police believe was used in Dugard's abduction -- looked very much like the car involved in Michaela's disappearance.
"To live with a hope that is continuously unfulfilled is really one of the most difficult things in the world," Murch said. "For a while it was easier for me to believe she was in a better place than this, where she is not in pain."
The other child, 13-year-old Ilene Misheloff, vanished from Dublin just months later, in January 1989, after taking a shortcut across a creek bed on her way home from school.
Her parents have not commented on the search in Antioch, but her father, Mike Misheloff, told ABCNews.com last month that the Dugard case had given hem hope.
Jaycee Dugard Case Turning Attention on Other Missing Children
Lowery said the national attention paid to the Jaycee Dugard case has been a good thing for the mission of National Center for Missing and Exploited Children.
"We have gotten more calls as a result on other missing children," he said.
Though no connections have ever been made between the Garridos and other disappearances, there are still several missing girls from Northern California.
They include Vanessa Smith, kidnapped at age 15 in 1997, and Melissa Espinoza, kidnapped at age 12 in 1993.
And while some people assume authorities give up after a child is still missing months or years later, Lowery said, "We never give up hope about finding children alive."
Phillip and Nancy Garrido have pleaded not guilty to a total of 28 counts in the Jaycee Dugard case, including kidnapping and rape. Garrido is being held on $30 million bail. His wife's attorney has not requested bail.
Dugard was kidnapped in 1991 near her school bus stop. Her stepfather, Carl Probyn, told ABCNews.com that he heard her scream and saw her being pulled into a car. He chased the car on a bicycle, but was never able to catch them.
After Dugard was found alive, Probyn said the description he gave police of the car involved in the kidnapping exactly matched the gray Ford taken off the Garridos' property.
Dugard and her two daughters, 11 and 15, have remained in a secluded location with her mother, Terry Probyn. She is said to be in fairly good health, though she looks much younger than her 29 years.
ABC News Correspondent Brian Rooney contributed to this report.