Dutch police have confirmed they're investigating a large stash of cash found at the remote farmhouse in Ruinerwold where a family was allegedly locked away for nine years.

Gerrit-Jan van Dorsten, the 67-year-old father of the five adult children who were discovered inside a hidden room at the property, was arrested on Thursday and charged with unlawful deprivation of liberty, as well as abuse and money laundering.

Josef B, the 58-year-old Austrian handyman who rented the house, has also been arrested and charged with deprivation of liberty and money laundering.

"We have reasons to suspect that the six persons involved did not stay in the house at Buitenhuizerweg of their own free will," police said in a statement, according to news.com.au.

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While police said they could not confirm the exact amount of money found on the premises, a spokesman told local media it was enough to appear suspicious.

"It's not a couple of tens and that's why we are investigating where that money comes from," the spokesman told local newspaper, De Telegraaf.

The case has baffled authorities since 25-year-old Jan Zon van Dorsten escaped from the farmhouse on Sunday and fled to a nearby pub, downing five beers before telling the owner he feared for his family and didn't want to go home.

"It was immediately apparent … that something strange was going on," owner Chris Westerbeek said.

Jan and his younger siblings, aged between 18 and 25, are now being cared for "in a safe place", according to police, as authorities continue to scour their living quarters for clues.

"These people may have lived in the house with each other, separated from society, since 2010. The situation discovered requires a careful approach … various experts have been engaged," police said.

Officers told local media the children could read and write, but may never have visited a doctor or dentist.

"It is clear that they do not naturally exhibit the same behaviour as you and I do. What that means must be clear," deputy police chief Janny Knol told De Telegraaf.

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Meanwhile, the Unification Church has confirmed Gerrit-Jan was briefly a member of its religious movement in the mid-1980s, but left in 1987.

In a statement, the church said it was "deeply alarmed to hear of the family being held in inhumane conditions" and claimed Gerrit-Jan had "suffered from mental health issues" when he left the organisation.

It confirmed his estranged brother, Derk van Dorsten, was still an active member of the church, but was unable to confirm any records of Josef B having ever been associated with the group.

"We are grateful that the six victims in this tragedy are now under the care of the local authorities and pray that they will be able to heal from their ordeal with time and professional help," it said.

The Unification Church has millions of followers around the world often — referred to as 'Moonies' — and is famous for its mass weddings.

But critics say it's a dangerous cult with powerful business interests, especially in the United States.

"We are investigating whether a certain religion or philosophy forms the cause of (the family's) living situation," police said.