The suspect was in his 30s and suspected of either murder or complicity in the death of Anne-Elisabeth Hagen, who went missing in October 2018, a police statement read.
Police said the suspect was an acquaintance of investor Tom Hagen, Anne-Elisabeth Hagen's husband.
Tom Hagen was arrested last week and has been in pretrial detention on suspicion of either murdering or being complicit in the murder of his missing wife.
He has denied involvement.
An appellate court ordered Hagen's release from pretrial detention on Thursday, but police have appealed to the Supreme Court.
Anne-Elisabeth Hagen went missing from their Lorenskog home in Akershus county, near Oslo. She remains missing 18 months later despite a massive search operation.
Initially, the case was treated as an abduction. However, in June, police said that she might have been murdered and they were working on the assumption that her disappearance was an attempt to cover up the suspected killing.
Police have said they based their hypothesis on a lack of proof of life and a long period without contact with the alleged kidnappers.
The ransom payment was requested in cryptocurrency, which is difficult to trace. It is not known if any ransom was paid.
Police said the man they arrested on Thursday was known for his knowledge of information technology and cryptocurrency.
Police said the suspect was an acquaintance of investor Tom Hagen, Anne-Elisabeth Hagen's husband.
Tom Hagen was arrested last week and has been in pretrial detention on suspicion of either murdering or being complicit in the murder of his missing wife.
He has denied involvement.
An appellate court ordered Hagen's release from pretrial detention on Thursday, but police have appealed to the Supreme Court.
Anne-Elisabeth Hagen went missing from their Lorenskog home in Akershus county, near Oslo. She remains missing 18 months later despite a massive search operation.
Initially, the case was treated as an abduction. However, in June, police said that she might have been murdered and they were working on the assumption that her disappearance was an attempt to cover up the suspected killing.
Police have said they based their hypothesis on a lack of proof of life and a long period without contact with the alleged kidnappers.
The ransom payment was requested in cryptocurrency, which is difficult to trace. It is not known if any ransom was paid.
Police said the man they arrested on Thursday was known for his knowledge of information technology and cryptocurrency.