SAN FRANCISCO (AFP) - Police searching near the home of a California man accused of abducting a schoolgirl and keeping her captive for 18 years said Monday they had found a bone at a neighbouring property.

Contra Costa Sheriff's Department spokesman Jimmy Lee said it was too early to tell if the fragment unearthed Sunday near the home of convicted rapist Phillip Garrido in Antioch, east of San Francisco, was animal or human.

Tests to determine the origin of the bone would take several weeks, Lee said, as teams of investigators continued to pore over the ramshackle home where Garrido held Jaycee Lee Dugard prisoner for nearly two decades.

Dugard, kidnapped as an 11-year-old schoolgirl in 1991, was found alive last week. It later emerged that Garrido, 58, had fathered two children with Dugard.

Garrido and his wife Nancy have pleaded not guilty to 29 felony counts including rape, kidnapping and false imprisonment.

Police are looking for evidence of links between Garrido and the murders of several prostitutes at an industrial area where he worked during the 1990s. Garrido's ageing father meanwhile was quoted by the New York Post as saying he believed his son was guilty of the murders.

"He was a sex addict - that was his problem," Manuel Garrido, 87, told the Post. "I believe my son killed the prostitutes."

As the police investigation continued, more details of Dugard's life as a prisoner began to emerge, with the San Francisco Chronicle reporting that she had helped her tormentor run a thriving printing business in recent years.

The newspaper quoted customers of the business as saying that they had believed Dugard, known to them as "Allissa," was Garrido's daughter.

Dugard, now 29, was described as a courteous and professional young woman who dealt with clients in telephone calls and emails while helping them place orders for business cards, flyers and posters, the Chronicle reported.

"(Garrido) told us up front he works with his daughter. He said Allissa did all of the graphic design and he did all of the printing," said JP Miller, who hired Garrido this month to help advertise his haulage company.

Miller told the paper he saw nothing to suggest that "Allissa" was in fact the blonde schoolgirl who had been snatched from a street near her South Lake Tahoe home and who was later forced to bear two children to Garrido.

Another customer who dealt with Dugard said he saw nothing out of the ordinary in the young woman.

"She was very professional, very polite, just like any other secretary or anyone you'd meet at a place of business," said Ben Daughdrill. 'If I was requesting something, he'd say he'd have his daughter send it over. He'd say, "I'll get Allissa right on that.'"

Experts say Dugard was likely to have suffered from Stockholm syndrome, a condition in which captives become sympathetic to their captors and say it is likely the woman will need years of treatment as she rebuilds her life.

Dugard's stepfather Carl Probyn said Monday that Jaycee was slowly coming to terms with her ordeal after being reunited with her mother and her half-sister. "It's a minute by minute thing," Probyn told ABC. "It's going to take years."

Probyn revealed that Jaycee's two children had finally been confronted with the truth about their father, Garrido.

"Jaycee had to explain to them, like two days ago, that she had been kidnapped. They didn't even know that. They are upset about this because that's their father and he's in jail."

Dugard was confined in a makeshift prison of sheds and tents in what police have described as a "backyard within a backyard" at Garrido's home in Antioch, around 50 miles (80 kilometers) east of San Francisco.

Photographs of the secret compound appeared in British newspapers over the weekend showing a squalid network of living quarters strewn with junk.