"We have reasons to suspect that the six persons involved did not stay in the house ... of their own free will," police said in a statement. "We are investigating whether a certain religion or philosophy forms the cause of their living situation."
Police raided the farm in a rural village 130 kilometers (80 miles) northeast of Amsterdam on Monday after one man left the building and raised the alarm. They found six people, all adults who said they belong to one family, in makeshift rooms in the farm, which was barely visible from the road behind a row of trees. The man who raised the alarm after walking into a local bar and ordering five beers for himself also says he belongs to the family.
Drone images of the farm showed a large vegetable garden on one side of a group of buildings and chopped wood stacked up along a fence. A 58-year-old man who rented the farm was arrested shortly after Monday's raid on suspicion of involvement in "deprivation of liberty and harming the health of others" and money laundering. He reportedly is an Austrian national and not related to the other people involved in the case.
On Thursday, police also detained the supposed father in connection with the same suspicions. While the six people on the farm and the man who raised the alarm say they are all from the same family — a father and six siblings all now young adults — police say they are still investigating their exact relationship as none of the siblings appears to have been registered with authorities.
The local mayor said at a press conference earlier this week that the mother of the family is believed to have died a long time ago. "This investigation revolves around a unique situation," police said. "These people may have lived in the house with each other, separated from society, since 2010."
Dutch media initially reported that the family may have feared the end of the world. On Thursday, there were reports that they may have been followers of the Unification Church, which critics regard as a cult.
Police said the six siblings are being cared for by specialists while a 30-strong team of investigators combs the house for evidence in the case, including taking detailed digital images of all the living areas.