According to current knowledge, the fire in a museum barn in Uetersen in the Pinneberg district was caused by deliberate arson.
Two different experts had ruled out a technical defect as the cause of the fire, the police said on Thursday.
The criminal police in Pinneberg therefore assume an intentional arson, said a spokeswoman.
The fire in the thatched museum barn caused property damage in the six-figure range on Sunday evening.
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Nobody was injured in the fire.
The police are still looking for witnesses who made suspicious observations in the vicinity of the crime scene before and during the firefighting work.