Norwegian police said Tuesday they had arrested a wealthy businessman over the disappearance of his wife 18 months ago, the latest twist in a case that has kept the Nordic countries on tenterhooks.

Tom Hagen, 70, was arrested as he was leaving his home for work on Tuesday, on suspicion of "murder or accessory to murder," commissioner Ida Melbo Oystese told a press conference.

Hagen's wife Anne-Elisabeth Hagen disappeared without a trace from their home in Lorenskog, east of Oslo, on 31 October, 2018.

A poorly written ransom note, containing threats and a demand for nine million euros in crypto currency, was found at the scene. Sporadic contact with the alleged kidnappers on digital platforms initially supported the theory that she had been the victim of a rogue abduction.

However, by June 2019, police were re-evaluating their leads and started examining whether it was a murder that had been disguised as a kidnapping. Tom Hagen is the 164th richest man in Norway with an estimated net worth of 1.9 billion Norwegian kroner (€168 million) in 2019, according to business magazine Kapital.

His lawyer said he denied the allegations.

"He forcefully maintains that he has nothing to do with it," Svein Holden told public broadcaster NRK.

Police did not present any concrete evidence to support their suspicions or speculate on a potential motive, but said the case "had traces of a clear, premeditated misdirection."

Question marks remain as to what police believe Tom Hagen's exact role in the disappearance to be, whether he had accomplices and the whereabouts of Anne-Elisabeth Hagen, now presumed dead.

Police said they had yet to interrogate the suspect.