“If you can imagine being 17 years old and appearing to be a 10-year-old, being chained to a bed, being malnourished and injuries associated with that, I would call that torture,” Perris police chief Greg Fellows told reporters.

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

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

“The long-term needs of these kids are going to be the psychological and psychiatric needs due to the prolonged periods of starvation and maltreatment,” said Sophia Grant, medical director of the child abuse unit at Riverside University Health System.

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

“I believe that they’re hopeful that life will get better for them after this event,” he said, adding they were now in a “very safe and secure environment.”

Booked on suspicion of torture and child endangerment, the parents’ bail was set at US$9 million each.

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

Police said there was no initial indication of sexual abuse, but cautioned that the investigation was still ongoing.

There was also no indication that either suspect suffered from mental illness, Fellows said, or that the children’s ordeal was linked to the family’s religious beliefs.

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

According to police, the family moved in 2014 from Texas to a middle class neighbourhood of Perris, 110km southeast of Los Angeles, home-schooling their children in their Spanish-style stucco house.

A grey van with tinted windows and three cars were parked outside the house, now surrounded by satellite transmission trucks and hundreds of cameras and journalists.

Their personalised number plates related to Disney Land – DL4EVER, DSLAND and 4EVERDL.

“They looked vampire pale and very skinny,” one neighbour, Kimberly Milligan, said of the few children she had seen. “They never had friends or family over.”

“A lot of things were strange, but not enough to call the police … There’s a special place in hell for people like that,” she said.

A Facebook page under the name of David-Louise Turpin – still visible Tuesday – includes pictures of the couple renewing their wedding vows, from 2011 to 2016, with the children present.

In the latest set, uploaded in April-July 2016, an Elvis Presley impersonator holds a microphone and poses with the family in a Las Vegas wedding scene.

Nine girls, all with long dark hair, wear matching fuchsia plaid dresses with white tights, while a baby girl is dressed in bright pink.

Three boys, their dark hair in bowl cuts like David Turpin, are dressed in suits with red ties.

The Elvis impersonator, who renewed the couple’s vows three times at his Elvis Chapel in downtown Las Vegas, said he got physically ill when he learned of the abuse.

“I really got sick to my stomach and couldn’t believe it,” Kent Ripley told the New York Daily News. “I was shocked and stunned and very saddened.”

He recalled the children as being “very soft-spoken and well-mannered,” and “very close” to one another.