By Alexandria Sage

SAN FRANCISCO (Reuters) - A California girl who was kidnapped at the age of 11 in 1991 has been found alive, having spent 18 years living in sheds and tents behind the home of her accused abductor, a convicted rapist who fathered two children with her, police said on Thursday.

Jaycee Dugard had been missing since she was snatched off a street by two people in a gray sedan while walking to a bus stop near her home in South Lake Tahoe, east of San Francisco, on June 10, 1991.

Dugard, now 29, was found after a parole officer for her accused kidnapper became suspicious, leading to a search of his home near the town of Antioch, about 100 miles southwest of where she was abducted.

Police say the search turned up a hidden "backyard within a backyard" at the home of registered sex offender and convicted rapist Phillip Craig Garrido, where Dugard and the two children were confined in a series of sheds and tents.

"She was in good health but living in a backyard for 18 years must take its toll," El Dorado County Undersheriff Fred Kollar told a news conference.

Carl Probyn, Dugard's stepfather, said on television he and her mother "cried for about 10 minutes" after they were told by authorities that she had been found alive.

BREAK IN CASE CAME TUESDAY

Garrido, 58, and his wife Nancy Garrido, 54, were arrested over Dugard's abduction and prosecutors said they were likely to be charged on Friday.

Authorities said Garrido was suspected of fathering two children with Dugard, girls now aged 11 and 15.

"None of the children had ever been to school, none had been to a doctor. They were kept in complete isolation in this compound," Kollar said.

While the case of Dugard's abduction had remained open for the past 18 years, police had never found a trace of the missing girl or the gray sedan until Tuesday, when Garrido tried to enter the University of California, Berkeley campus with the two girls to pass out religious leaflets.

A campus police officer found his interaction with the girls suspicious and investigated his background, ultimately alerting his parole officer.

During a visit with the parole officer, Garrido brought his wife, the two girls and a woman identified as "Allissa" -- who proved to be Dugard.

Authorities said Garrido had served time in a Nevada prison for a 1971 kidnapping and rape conviction.

The San Francisco Chronicle newspaper reported Garrido was described as strange by his neighbors, who said he conducted religious revivals in a tent and claimed to have invented a device to control sound with his mind.

Asked by reporters why the parole officer, who had visited Garrido's home, had never found Dugard or the secret backyard compound, authorities said it was well hidden behind a fence, vegetation and a tarpaulin.

(Writing by Dan Whitcomb; Editing by John O'Callaghan)