SAN FRANCISCO (AFP) - Questions mounted Saturday about how a California man was able to hide for 18 years a girl he kidnapped and the two children she bore him, despite warnings from neighbors of something amiss. As scores of police combed the home of suspects Phillip Garrido, 58, and his wife Nancy, 54, Americans were asking how police failed to act on tipoffs that something suspicious was going on at their house. Jaycee Lee Dugard was discovered on Wednesday, nearly two decades after the blonde schoolgirl was snatched outside her home in 1991, when she was just 11. She had been confined in a makeshift prison of sheds and tents in what police have described as a backyard within a backyard at Garridos home in Antioch, around 50 miles (80 kilometers) east of San Francisco. On Saturday, police were searching the house in relation to a series of prostitute killings in the 1990s, as other bodies had been found close to where Garrido worked, the San Francisco Chronicle said. On Thursday, police revealed that Garrido, a convicted rapist and registered sex offender, had abused Dugard, now 29, and fathered her two daughters, now aged 15 and 11, who had also been kept in the compound. The Garridos both pleaded not guilty on Friday to 29 alleged offenses including kidnapping, rape and false imprisonment. But many are questioning how the case went unsolved for so long, even after neighbors alerted police that children appeared to be living in the complex of tents behind his home. Police in Contra Costa County admitted Friday that they had received a tip in November 2006 and failed to follow it up properly. Sheriff Warren Rupf issued an apology saying law enforcement officials were distraught over their failure to discover Garridos crimes earlier. On November 30, 2006 we missed an opportunity to bring earlier closure to this situation, he admitted. I cant change the course of events, but we are beating ourselves up over this and are the first to do so, Rupf said. But other neighbors said they had no idea that anything was wrong. Its kind of embarrassing to be here this long and not know whats going on. How could that go on under all of our noses? one neighbor, who gave his name only as Steve, told AFP. A man who once hired Garrido for a printing job told The New York Times on Saturday that he had met, exchanged emails and regularly spoken on the phone with a young woman who was introduced as Garridos daughter Allissa. Ben Daughdrill said the woman never suggested she was being held captive or tried to identify herself as Dugard, and she was the one who did the art work. Dugard was reunited with her mother and half-sister on Friday, but was struggling to come to terms with her ordeal and experts say it could take years for her to recover. Dugard was found after police reported Garrido acting suspiciously at the University of California, Berkeley, where with two young girls he was trying to hand out religious literature propounding claims he could channel the voice of God. The pale, blond girls were wearing drab sun dresses, like 'Little House on the Prairie meets robots, university security officer Allison Jacobs told a press conference Friday. The younger daughter was staring directly at me, as if she was looking into my soul, with this eerie smile on her face, Jacobs said. I just got a weird, uneasy feeling. Garrido apparently boasted to the campus officers: Im so proud of my girls. They dont know any curse words. They dont know anything bad about the world. Garrido was summoned to a meeting Wednesday with his parole officer who, having previously visited the home, found it strange that in addition to his wife Nancy he brought along two girls and a woman he called Allissa. Dugards real identity emerged during the meeting and Garrido and his wife Nancy were detained. But parole officers Saturday defended their work, saying Garrido had taken pains to cover his tracks. This guy was definitely very elusive, very stealthy in what he was doing, Gordon Hinkle, spokesman for the state parole agency, told the Chronicle.