The Southern California home where authorities say two parents tortured their 13 children had doubled as a private school for the siblings but faced no government oversight and was never inspected by education officials.
David Turpin had been home schooling his children at the residence, which he called the Sandcastle Day School. In the 2016-17 school year, it had an enrollment of six, with one student each in the fifth, sixth, eighth, ninth, 10th and 12th grades.
There was no indication anyone other than his children were enrolled, authorities said.
Authorities said they uncovered the abuse when a 17-year-old girl jumped out of a window, called 911 and led police to 12 siblings from 2 to 29 years old, all malnourished and some chained some to furniture.
Turpin and his wife, Louise Anna Turpin, were being held on $9 million bail and could face torture and child endangerment charges.
Susan von Zabern, director of the county's social service department, said it was "the first opportunity to intervene."
Private schools in California are not licensed by the state education department and no agency regulates or oversees them. The schools are only required to file an affidavit with the state each year that lists the number of students, staff members and information about administrators.
"We are sickened by this tragedy and relieved the children are now safe and authorities are investigating," Bill Ainsworth, a spokesman for the California Department of Education, said in a statement.
Full-time private schools must register with the state to record their students' exemption from mandatory attendance at public schools, but the education department lacks the authority to monitor, inspect or oversee private schools, Ainsworth said.
They are, however, subject to an annual inspection by the state or local fire marshal. Representatives for the California Department of Forestry and Fire Protection, the Riverside County Fire Department and the Perris Fire Department did not immediately say whether the Turpins' home was ever inspected.
Private schools are generally regulated by parents, who often sign contracts and review standards before enrolling their children, said Ron Reynolds, executive director of the California Association of Private School Organizations. Private school employees are required to report suspected child abuse to authorities, he said.
Asked whether the organization, comprised of 1,500 private schools in California, believes oversight is sufficient, Reynolds said, "We always desire as our first priority safeguarding our children. We never resist or oppose legislation aimed at protecting students."
But Reynolds questioned how it's possible to prevent parents who home-school their children from injuring their kids in the privacy of their own houses.
"Thank God this doesn't happen very often," he said.
Associated Press writer Amy Taxin in Perris contributed to this report.