THE HAGUE/AMSTERDAM• • Dutch police are holding an Austrian man after the discovery of a father and his adult children who were believed to have stayed hidden in the cellar of a remote farmhouse in the Netherlands for years, officials said yesterday.

The mystery surrounding the case also deepened with reports that one of the children had been active on social media this year.

Five siblings, said to be aged 18 to 25, and a man they identified as their ailing father were found near Ruinerwold, a village in the northern province of Drenthe.

Police said they discovered the family on Monday and arrested a 58-year-old man - not the father - for failing to cooperate. They initially spoke of six children but later revised the number down.

The family members were living in a "small space in the house which could be locked", said the police on Tuesday. "It is unclear if they resided there voluntarily."

Yesterday, police said they were questioning a man who paid the rent on the farmhouse, but could not say whether he was suspected of a crime.

The Austrian Foreign Ministry confirmed an Austrian national from Vienna was being held in relation to the case, but said the man did not want to have contact with Austrian officials.

Ministry spokesman Peter Guschelbauer told Agence France-Presse the exact grounds for the man's arrest are not known. The Dutch authorities said all others in the case were Dutch, he added.

Dutch and Austrian media said the man was known as "Josef the Austrian" and was the one holding the group in the cellar, renting the house and planting vegetables for them in the garden.

"The man is still in custody and is being questioned," said Drenthe police spokesman Grietje Hartstra.

"A lot is still unclear and we are investigating exactly what happened there."

According to media reports, the family had spent years "waiting for the end of time", with Ruinerwold's mayor saying: "I have never come across anything like this before."

The oldest of the siblings, a 25-year-old who is known as Jan, kept a Facebook account and, for the first time in nine years, posted an update in June, according to Dutch media. "Started a new job at Creconat," daily tabloid De Telegraaf quoted him as saying.

Creconat is affiliated to a company in the nearby town of Meppel and was raided by the police on Monday, De Telegraaf said, adding that it belonged to the Austrian man.

Jan also posted pictures of himself "surrounded by trees" and in and near Ruinerwold, as well as links to products and rallies including the recent climate marches, the paper said. On his LinkedIn page,he wrote that his parents had run a successful business until his mother died in 2004.

Local news station RTV Drenthe said the police were alerted after Jan walked into a village bar on Sunday evening and said he had not been outside for nine years.

"You could see he had no idea where he was or what he was doing," cafe owner Chris Westerbeek told RTV Drenthe. "He said he had run away and that he urgently needed help."

Police went to the house and, upon investigation, discovered a hidden staircase behind a cupboard leading to a cellar where the family had been hiding.

AGENCE FRANCE-PRESSE, REUTERS