To the outside world she was plain, modest Nancy, the other half of Phil the printer. Neighbours knew her as a full-time carer for Phillip Garrido’s ageing mother and a part-time accountant for his printing business.

In reality Nancy Garrido provided the cover that helped her husband, a convicted sex attacker, to keep the authorities at bay. When neighbours spotted children at the house they reasoned that it must be OK because he lived with his wife.

But Mrs Garrido, 55, is facing life in prison for being her husband’s alleged partner in nearly two decades of child rape, imprisonment and abuse. Police believe that he could not have done it without her.

Prosecutors have given her equal billing on the 16-page charge sheet. She is accused of forcible rape, committing a forcible lewd act upon a child and of false imprisonment by violence, among 29 other charges dating from the day that Jaycee Lee Dugard was snatched from South Lake Tahoe, in California, in 1991.

Police believe that Mrs Garrido was the woman who grabbed the 11-year-old girl and have pointed to how similar she looks to the sketch of a dark-haired, female abductor produced by Jaycee’s stepfather, Carl Probyn, who witnessed the kidnapping.

Mrs Garrido and her husband pleaded not guilty to the charges when they appeared in court last week. She looked distraught throughout much of the short hearing, bowing her head and weeping several times.

Police suspect Mr Garrido may have been involved in other crimes, and are now searching his home for links to other murders and missing children.

It also emerged that dozens of men with convictions for sex offences against children live within walking distance from the property. Mr Garrido’s 94509 ZIP Code is home to more than 100 registered sex offenders, according to a register of sex offenders.

Phillip Garrido’s father, Manuel Garrido, said he believed that his son was a serial killer. "He was a sex addict. That was his problem," Manuel Garrido told The New York Post. “I believe my son killed the prostitutes.”

Detectives believe that Mrs Garrido is the key to unravelling what went on during the 18 years that Jaycee was kept in the hidden compound behind the Garrido house in Antioch. They fear that Garrido, 58, whose mental health is clearly an issue, may prove more difficult to pin down.

Mrs Garrido’s role has stunned America, leading to speculation about her motives, her relationship with her husband and her own mental state. Little has been made public about her early life but it is believed that she was a Jehovah’s Witness when she met Garrido during a visit to her uncle in a prison in Leavenworth, Kansas.

Garrido was serving time for abducting a 25-year-old woman and raping her in a storage unit that he had converted into a “sex palace” but Mrs Garrido fell for him and they began writing to each other. They were married in a prison ceremony.

On his release in 1988 the couple moved into his mother’s house in Walnut Avenue, Antioch. Helen Boyer, a neighbour, occasionally went over for dinner in the first few years.

Ms Boyer, now 78, never knew Mrs Garrido well but said that she had always thought of her as a “caring person”. “She looked after [Garrido] and his mother and then, when the mother became bedridden with dementia, she did all the caring,” she said.

Ms Boyer said that she knew of their past but had no inkling of the horrors that went on a few yards away. “I’d holler ‘hello’ at her and she would holler back. We had no idea at all. They should throw the book at them,” she told The Times.

Police particularly want to question Mrs Garrido about the four months in the summer of 1993 when her husband went back to prison for breaking parole. Jaycee was 13 and, according to court documents, already the victim of multiple rapes at the hands of both Mrs Garrido and her husband.

The timing is unclear but it was about this time that Jaycee became pregnant with her first daughter, now 15. What hold did Garrido have on his wife so that, even with him in prison, Jaycee did not escape? Or was Mrs Garrido much more than simply the little girl’s jailer?

Psychologists have speculated that Mrs Garrido was a victim of catastrophically low self-esteem and, once under her husband’s spell, would do anything to hold on to her man.

After she had obtained a victim for him, her anger at having a rival for his affections could have driven her to even greater depths of cruelty and abuse.

A 2007 study for the US Department of Justice of female sex offenders found that there was a type of offender labelled “male-coerced”. It found that these women “tended to be passive and dependent individuals with histories of sexual abuse and relationship difficulties. Fearing abandonment, they were pressured by male partners to commit sex offences”.

Garrido’s brother, Ron, said that Mrs Garrido was a “robot”. “She would do anything he asked. I told my wife, ‘It’s no different from [Charles] Manson and those girls’. She was under his control.”

Police believe that Mrs Garrido helped Jaycee to give birth to her two daughters, to raise them and to keep them quiet. She created a second family for her husband, for whom she did not produce any children of her own, and became their mother figure.

She was at her husband’s side on Wednesday last week when they went to see his probation officer — a meeting that finally led to their arrest.

Now they face the rest of their lives in separate prison cells.

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