Garrido and his wife, Nancy, were arrested last week and charged with 29 counts connected to the kidnapping, rape and imprisonment of Jaycee Lee Dugard, who was snatched outside her home in South Lake Tahoe in 1991. They have pleaded not guilty.
The bone fragment was found Sunday in the next door neighbor's backyard, said Contra Costa County Sheriff's Department spokesman Jimmy Lee. He said it is not known if it is from an animal or human, and testing will take several weeks.
Garrido once lived on that property in a shed. Neighbors say he once worked as the property caretaker and helped out an elderly man who lived there several years ago.
Antioch and Contra Costa County authorities have not provided many details about what they are seeking at the properties in relation to the other cases. The land includes the backyard compound of tents and sheds where Garrido and his wife allegedly hid Dugard and her two daughters, now 11 and 15, fathered by Garrido.
Police in the nearby city of Pittsburg, however, have said they are investigating whether Garrido, 58, was involved in the murders of prostitutes in the 1990s.
Cadaver-sniffing dogs have been on and off the properties to help uncover possible evidence.
Meanwhile, an FBI agent who spent 18 years on Dugard's kidnapping case says the Garridos never were considered suspects.
Special Agent Chris Campion said the bureau exhausted thousands of leads about Dugard's whereabouts, sometimes with the help of confidential informants and court-ordered wiretaps.
Yet Campion said in the interview posted on the FBI Web site Friday that Phillip and Nancy Garrido "just did not come up on the radar screen."
"We've gone through and checked our records and my memory is no, we didn't have any thing that remotely was close to these people," Campion said. "We can tell you several thousands of people that didn't kidnap Jaycee Lee Dugard."
The secrets of the Garrido home began to surface early last week when Garrido arrived for a meeting with his parole officer with his wife, Dugard, now 29, and the two girls. Authorities say he confessed to snatching Dugard in 1991.
Over the years, Campion said he made a point of calling Dugard's mother every year on Dugard's birthday. He was the one who called to give her the news that her daughter was alive and he was present last week when they were reunited.
"It was a very emotional scene - both of them were just overjoyed to be with each other again," he said. "There's going to be a period of adjustment, no doubt, but they're doing very well at this point. And the two daughters are probably as happy as Jaycee is to be part of this family as well."