Neighbours of the Turpin family in Perris, California, have described their odd behaviour and "heartbreak" at not realising what was happening in the middle of their community.
On Sunday, a 17-year-old girl escaped from the Turpin family home more than 100km southeast of Los Angeles and phoned police on a mobile.
Police entered the house to find her 12 siblings locked up in filthy conditions with some shackled to their beds under padlocks and chains.
The Riverside County Sheriff's department said they didn't realise many of the emaciated children were actually adults, with the group ranging from two to 29 years old.
The parents, David Allen Turpin, 57, and Louise Anna Turpin, 49, were arrested on suspicion of nine counts of torture and 10 counts of child endangerment. They are being held on bail set at US$9 million ($12m) each.
Neighbour Andria Valdez said her she was shocked to learn what had been happening under the locals' noses. She said her family had previously joked the Turpins reminded them of the Cullen family vampires from the Twilight books and films.
"They only came out at night," she told the Press Enterprise. "They were really, really pale."
"The older boy would come out, like all pale and he would have like a phone and he would record, like, the plants," she said in a separate interview with local media.
"I was like 'what's wrong with this kid?' You know, 'what's going on with them?'"
"It's heartbreaking. Now to know that there were [that many] kids in there and to not know about it then. I wish there was something the community could have done."
Zinzi and Ricardo Ross who had lived on the street for more than two years said they couldn't believe what had happened.
"It's very shocking," Ricardo Ross said. "Very devastating."
"It's crazy. I can't believe this is going on," Zinzi Ross said.
Robert Perkins said he and his mother saw a few family members constructing a nativity scene in the Turpins' garden a few years ago. Perkins said he complimented them on it, but "they didn't say a word".
US education records show the family had a home school listed at the address. The family is also reported to have filed for bankruptcy in 2011 with debts of between $100,000 and $500,000.
The Turpin's joint Facebook page shows pictures of a large and seemingly loving family of 13 children dressed in the same outfits and with matching haircuts. Pictures of David and Louise show them renewing their wedding vows and staring lovingly into each others' eyes with comments like "lovely couple" underneath.
David's mother Betty Turpin told CNN they were a "highly respectable" family who usually took the whole family on holiday and dressed alike for safety reasons.
"They were very protective of the kids," she said. When they went out, the couple would line the children up according to age, and the parents took their positions at the front and back of the line, she said.
"It was easier to keep up with the kids that way," she said.
However the Riverside County Sheriff's report paints a chilling picture of what happened inside the home. It said police found "several children shackled to their beds with chains and padlocks in dark and foul-smelling surroundings".
"The parents were unable to immediately provide a logical reason why their children were restrained in that manner."
Police also noted the children appeared emaciated, and they had believed the 17-year-old to be around 10.
The children have all been taken to hospital for examination while the parents are being held at a local detention centre.