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 that children were living in a complex of tents in his backyard.

For 18 years Phillip Garrido eluded detection as he pulled off what authorities are calling an unfathomable crime, kidnapping and raping then-11-year-old Jaycee Dugard, keeping her as his secret captive for nearly two decades and fathering two of her 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 that he was a serial killer as well.

Several of the murdered women's bodies, the exact number is not known, were dumped near an industrial park where Garrido worked during the 1990s.

Authorities acknowledged that they blew a chance three years ago 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 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," 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.

Dugard, now 29, 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, Nancy Garrido, pleaded not guilty Friday to a total of 29 counts, including forcible abduction, rape and false imprisonment.