(Corrects second last paragraph to reflect that Garrido, not Dugard, was arrested)

SAN FRANCISCO (Reuters) - Authorities investigating a man accused of kidnapping an 11-year-old girl and keeping her in his backyard for 18 years searched his California home on Friday for evidence in the 1990s murders of prostitutes.

Police apologized for failing to follow up an anonymous tip in 2006 that could have led them to Jaycee Dugard, who was snatched from a bus stop near her South Lake Tahoe home in 1991.

Authorities believe that the now 29-year-old Dugard spent most of the past two decades living in squalid tents and sheds in the backyard of convicted rapist Phillip Garrido, who is accused of fathering two children with her.

Police searched Garrido’s home on Friday for evidence that might also connect him to killings of prostitutes in the 1990s the area around his home near the Bay Area suburb of Antioch.

“They’re out there right now. They’re searching the house again,” said Jimmy Lee, spokesman for the Contra Costa Sheriff’s Department.

Contra Costa Sheriff Warren Rupf acknowledged that a neighbor of Garrido called police in 2006 and said he was a “psychotic” with sexual addictions and had children living in tents in his backyard.

When an officer went to the home to follow up, he spoke with Garrido in the front yard, warning him there were restrictions against living outside in a residential neighborhood, Rupf said.

“He did not enter, nor request to enter the backyard,” Rupf told reporters, adding that the officer did not know that Garrido was a registered sex offender, even though he is listed as such on a public database.

“This is not an acceptable outcome,” Rupf added. “Organizationally we should have been more inquisitive or curious and turned over a rock or two.”

Garrido, who served time in prison for rape and kidnapping in the early 1970s, made his first appearance in court on Friday. He wore an orange jumpsuit and sat next to a female attorney.

His wife, 54-year-old Nancy Garrido, is also in custody. Police have said that she matches the description of a woman riding in the car into which Dugard was pulled when she was abducted.

Authorities say Garrido kept Dugard and the two daughters he fathered with her in the backyard of the home he shared with his wife. The yard was surrounded by tarps and tall trees, making it difficult for outsiders to see inside, they say.

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Dugard was found this week after she showed up with Garrido for a meeting with his parole officer.

The appearance of Dugard and her 11- and 15-year-old daughters, all of whom the parole officer had never before met, caused suspicion and Garrido was arrested soon thereafter.

Suspicions first arose the day before, when Garrido was seen on the campus of University of California at Berkeley passing out leaflets with the two children.