ANTIOCH, Calif. — His neighbors knew he was a registered sex offender. Kids on his block called him "Creepy Phil" and kept their distance. Parole agents and local law enforcement regularly visited his home and found nothing unusual, even after a neighbor complained children were living in a complex of tents in his backyard.

For 18 years, Phillip Garrido managed to elude detection as he pulled off what authorities are calling an unfathomable crime, kidnapping 11-year-old Jaycee Dugard, keeping her as his secret sex slave for nearly two decades and fathering her two children.

The question about how he went unnoticed became more pressing Friday when Garrido came under suspicion in the unsolved murders of several prostitutes, raising the prospect he was a serial killer as well. Several of the women's bodies were dumped near an industrial park where Garrido worked during the 1990s.

And authorities acknowledged that they blew a chance in 2006 to rescue Dugard from the backyard labyrinth of sheds, tents and outbuildings that were concealed from the outside world.

A neighbor called 911 in November 2006 and described Garrido as a psychotic sex addict who was living with children and had people staying in tents in his backyard.

The investigating officer spent a half-hour interviewing Garrido on his front porch but did not enter the house or search the backyard, Contra Costa County Sheriff Warren E. Rupf said. The deputy, who did not know Garrido was a registered sex offender even though the sheriff's department had the information, warned Garrido that the tents could be a code violation before leaving.

"We missed an opportunity to bring earlier closure to this situation," Contra Costa County Sheriff Warren E. Rupf acknowledged. "I cannot change the course of events but we are beating ourselves up over this and continue to do so."

"We should have been more inquisitive, more curious and turned over a rock or two."

It was not the only missed opportunity.

As a parolee, Garrido wore a GPS-linked ankle bracelet that tracked his every movement, met with his parole agent several times each month and was subject to routine surprise home visits and random drug and alcohol tests, California Department of Corrections and Rehabilitation spokesman Gordon Hinkle said.

The last unannounced visit by a team of local police agencies was conducted in July 2008. Paramedics also were summoned to the house five times since 1999, presumably to help Garrido's 88-year-old mother.

"There was never any indication to my knowledge that there was any sign of children living there," Hinkle said.

As it turns out, Dugard and her two children were living there as virtual prisoners, authorities say. The compound was arranged so that people could not view what was happening, and one of the buildings was sound-proofed and only accessible from the outside.

Dugard was reunited with her family and said to be in good health but feeling guilty about developing a bond with Garrido over the years. Her two children, 11 and 15, remained with her.

Garrido and his wife pleaded not guilty Friday to a total of 29 counts, including forcible abduction, rape and false imprisonment. Phillip Garrido appeared stoic and unresponsive during the brief arraignment hearing. His wife cried and put her head in her hands several times.

Monica Adams, 33, whose mother lives on their street, said she knew Phillip Garrido was a sex offender and that he had children living with him. Other neighbors knew, too, but they assumed police were keeping tabs on him.

"He never bothered any one, he kept to himself," Adams said. "What would we have done? You just watch your own."

Some of the answers won't come until Dugard tells her story. The authorities say they do not yet know whether she ever tried to escape or to alert anyone of her whereabouts.

She had chances to escape. Garrido took her and the children around town with him, and he did a stint behind bars during the period of captivity.

A violation of Garrido's parole conditions sent him back to federal prison from April to August of 1993. Dick Carelli, spokesman for the federal Office of Court Administration, couldn't immediately tell from old records what Garrido did to violate parole.

Authorities are trying to piece together how and by whom Dugard was held during Garrido's four-month absence.

The discovery, which came to light after Garrido showed up at a parole office this week with his wife and three alleged victims in tow, had many people in northern California agonizing over how the kidnapped girl who grew into a 29-year-old mother could have been hidden in plain sight all these years.

"There are absolutely no excuses, and organizationally we should have had a better result," the sheriff said. "We took things he said obviously at face value. We did not properly brand him and our immediate goal is to fix that."

Garrido was required to register as a sex offender because he was convicted in 1977 of kidnapping a 25-year-old woman from parking lot in South Lake Tahoe, the same town Jaycee Dugard lived in when she was snatched from a school bus stop.

In the 1976 rape, Garrido took the woman across the state line into Nevada, where he raped her in a mini-warehouse in Reno that had been furnished with rugs, pornographic magazines and sex toys, according to prosecutors and news accounts from the time.

Gail Powell, spokeswoman for the Nevada Department of Public Safety, said Garrido met his wife while he was serving time for the rape at the federal penitentiary in Leavenworth, Kan.

He served about 10 years of a 50-year federal sentence for kidnapping and less than a year for a concurrent Nevada sentence of five years to life in prison for sexual assault. He was paroled in 1988 from the Northern Nevada Correctional Center, said Nevada Department of Corrections spokeswoman Suzanne Pardee.

He was supervised on parole by federal and Nevada parole agents until 1999, when a California parole agent took over under an interstate agreement, Hinkle said.

Because he was a sex offender, his parole agent had a lighter caseload: 40 convicted rapists and child molesters, down from a typical caseload of 70 or more for other ex-convicts. He held a job and never had a parole violation in the 10 years he was watched by California authorities, officials said.

"When the home visits were done, there was nothing abnormal or anything to raise suspicion," said Hinkle.

Hinkle said the alarms raised by the neighbor who contacted the sheriff's department never were relayed to Garrido's parole agent. But there was no ban on him having contact with children, nor restrictions on his travels.

"There was never any indication to my knowledge that there was any sign of children living there," Hinkle said.

Hinkle said Garrido's parole agent was shocked Tuesday when University of California, Berkeley, police told him that the man he had been monitoring for years had been seen with two small children.

The agent, whom officials refused to name or make available for interviews, called Garrido into his office the next day. Garrido arrived with his wife, the children and a woman who initially identified herself as Allissa. She turned out to be Dugard and investigators said Garrido confessed to the kidnapping.