OSLO (REUTERS) - The millionaire husband of a Norwegian woman thought to have been abducted and held for cryptocurrency ransom in 2018 was arrested on Tuesday (April 28) on suspicion of killing her, police said.

Ms Anne-Elisabeth Hagen, then 68, vanished from her home in October 2018, in a case that has fascinated the Nordic nation since police first went public with the news in January 2019.

Kidnappings and murders are rare in Norway, which prides itself on low crime rates.

The Hagen family had said it had received demands for ransom to secure her release - reported by Norwegian media to amount to some US$10 million (S$14.17 million) in a cryptocurrency known as Monero.

On Tuesday, Norwegian police said it had arrested her husband, businessman Tom Hagen.

"After now 18 months of investigation, police have come to a point where it has reason to suspect Tom Hagen of murder or conspiracy for murder," police lawyer Aase Kjustad Eriksson told a news conference.

Police inspector Tommy Broeske told the same news conference: "There was no kidnapping, no real negotiating counterpart or real negotiations. There are indications of a will to sidetrack (investigators)."

Mr Hagen did not respond on Tuesday to Reuters' attempts to contact him. A lawyer for the family, Mr Svein Holden, could also not immediately be reached for comment.

Norwegian media said Mr Hagen had been arrested on his way to work earlier on Tuesday.

In Norway being held under suspicion of a crime is a legal status that allows police to make an arrest and for a suspect to get a lawyer.

Formal charges, if brought, come much later in the legal process, sometimes only weeks before a court case begins.

SECRET CASE

In 2018 and 2019, fearing for Ms Anne-Elisabeth's safety after her disappearance, police had initially kept the case secret but eventually decided to go public in the hope that this would bring more leads.

A week after the police went public, the alleged kidnappers again contacted the family via a digital platform, Mr Holden, the family lawyer, said at the time.

"The family has not seen proof that Anne-Elisabeth is alive nor that the people who claim to be in control of Anne-Elisabeth actually do have her now," Mr Holden said then.

The lawyer declined at the time to reveal the exact contents of the communication, adding that the digital platform on which it took place offered very little opportunity for dialogue.

Norwegian business magazine Kapital has estimated the net worth of Tom Hagen to amount to around US$200 million.