A bone fragment pulled from a backyard next to the northern California home where Jaycee Lee Dugard was allegedly held captive for 18 years is probably human, a sheriff's spokesman said Tuesday night.

The assessment was made by a forensic expert who performed tests on the bone recovered last week at a home where Dugard's alleged kidnapper, sex offender Phillip Garrido, once stayed, said Jimmy Lee of the Contra Costa County Sheriff's Department.

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But Lee said it is not unusual to recover Native American remains in the area. The department has asked the state crime lab to perform DNA tests to determine the age of the bone and who it might have belonged to.

Investigators discovered the shard in the yard of the home next to the one where Garrido and his wife, Nancy, allegedly held Dugard captive for 18 years.

For a brief time in 2006, Garrido took care of the neighboring house while it was vacant and sometimes camped in the yard.

Police say Garrido snatched Dugard as she walked to a school bus stop near her South Lake Tahoe home in 1991.

He allegedly raped her repeatedly and fathered her two children during the 18 years he held her in a hidden backyard compound of tents and sheds behind his home in Antioch, Calif.

Phillip and his wife Nancy Garrido are behind bars and have pleaded not guilty to kidnap, rape and imprisonment charges.

The Dugard family's location has been a closely guarded secret since Jaycee, now 29, reappeared two weeks ago.

The Associated Press contributed to this report.