A California couple pleaded not guilty Thursday to multiple counts of torture as grisly details emerged of how they allegedly kept their 13 children in locked rooms or chains, allowing them to shower no more than once a year.

David Allen Turpin, 57, and his wife Louise Anna Turpin, 49 -- who had registered their home as a school -- were hit with 12 counts of torture, 12 of false imprisonment, six of child abuse and six of abuse of a dependent adult ahead of their court appearance in the city of Riverside.

The couple arrived in court dressed in black with their hands and legs shackled, and were represented by a public defender.

David Turpin was also charged with committing a lewd act against a child by force or fear or duress, District Attorney Mike Hestrin told a press conference.

The court set bail at $12 million for each of the defendants.

"If convicted of all charges, they face 94 (years) up to life in prison," Hestrin told reporters in Riverside.

Sheriff's deputies in Perris, a town southeast of Los Angeles, found three of the captives had been shackled with chains and padlocks in their filthy, foul-smelling home Sunday after receiving an emergency assistance call from their teenage sister who had managed to escape.

Hestrin said the 17-year-old had been working on a plan to escape for more than two years, and took one of her siblings with her, who became frightened and turned back.

The teenager was so emaciated that officers first thought she was a young child.

Officers also initially assumed all the other siblings to be children, but were shocked to discover seven ranging in age from 18 to 29.

All 13 are being treated for malnutrition and undergoing other diagnostic tests.

- 'Prolonged abuse' -

Hestrin said all the children had been subjected to "prolonged abuse," were not allowed to shower more than once a year, and barred from seeing a dentist or doctor.

"Circumstantial evidence in the house suggests that the victims were often not released from their chains to go to the bathroom," he told the press conference.

"If the children were found to wash their hands above the wrist area, they were accused of playing in the water and they would be chained up," Hestrin said.

When they were not chained up, they were locked in different rooms and were not allowed to have toys, "although there were many toys found in the house that were in their original package and had never been opened."

While the children's ordeal began when the family was living in the Fort Worth region of Texas, it "intensified over time and worsened" when they moved to California.

"They were fed very little, on a schedule," Hestrin added.

Mark Uffer, chief executive officer at the Corona regional medical center where the adults were being treated, has described their condition as stable.

Neither parent was able to immediately explain why their children were restrained, according to the Riverside County Sheriff's Department.

There was no indication that either suffered from mental illness, Perris police chief Greg Fellows said, or that the children's ordeal was linked to the family's religious beliefs.

- 'Begged to see them' -

Initial investigations have confirmed that the couple were the biological parents of all 13 siblings.

According to police, the family moved in 2014 from Texas to a middle class neighborhood of Perris, some 70 miles (110 kilometers) southeast of Los Angeles, homeschooling their children in their Spanish-style stucco house.

A sister of Louise Turpin, Elizabeth Flores, told ABC the couple kept to themselves.

"This has been going on before they even had children... they were real private, and they didn't come around much," said Flores. "We begged to Skype them. We begged to see them."

As a university student Flores lived with the Turpins for a while.

"I thought they were really strict, but I didn't see any abuse," Flores said.

But she said she did have disturbing memories of the husband. "If I went to get in the shower, he would come in while I was in there and watch me. It was like a joke. He never touched me or anything."

The case recalls previous kidnapping horrors that have made global headlines in recent years.

Ariel Castro abducted three young women he repeatedly raped for a decade at his Cleveland home. He was arrested in May 2013 after one of his victims escaped.

Jaycee Dugard was kidnapped as an 11-year-old and repeatedly raped over 18 years by convicted sex offender Phillip Garrido in California. She was rescued in August 2009.

Austria has seen two high profile kidnaps -- Elisabeth Fritzl was imprisoned and raped over a period of 24 years by her father Josef while Natascha Kampusch was held for eight years by Wolfgang Priklopil before her 2006 escape.

Source: AFP