The wife of one of Norway’s richest men was kidnapped more than two months ago and her captors are demanding a ransom for her release, police have announced.

Investigators say 68-year-old Anne-Elisabeth Falkevik Hagen has been missing since 31 October last year, after disappearing from her home in Lørenskog, just outside Oslo.

Her husband, Tom Hagen, a property mogul and owner of Scandinavian power giant Elkraft AS, is regarded as one of Norway’s wealthiest people.

Worth an estimated 1.7bn krone (£157m), he placed 172 on a list of the country’s 400 wealthiest people published by financial magazine Kapital.

Police said a note found in the couple’s house described what would happen to her if the ransom was not paid in the cryptocurrency Monero.

Detectives did not disclose how much money the kidnappers had asked for, but Aftenposten, Norway’s largest newspaper, reported they had asked for €9m (£8m).

The newspaper also said police had found no sign of a break-in at her home, but suspected she had been attacked in the property’s bathroom.

Reports in Norway have also speculated the ransom note stipulated Ms Falkevik Hagen would be killed if police became involved.

Chief investigator Tommy Broeske declined to comment on specifics surrounding the contents of the note, telling a press conference: “the threats were of a very serious character”.

“The reason for us to go public with this case now is that despite a broad and extensive investigation, we need more information,” he added. “We have no suspects in this case”.

Mr Broeske said “those behind have chosen to communicate digitally and we have had no other type of contact”. Police have urged the family not to pay any ransom.

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Investigators said they were working with Europol and Interpol on the case.

Kidnappings are rare in Norway, which prides itself on its low crime rates. It is believed the case is the first in the country’s history where captors have asked for a ransom in cryptocurrency.

Monero, the currency requested by the kidnappers, is known for its high levels of privacy, which in the past have presented law enforcement agencies with difficulties determining the source, amount or destinations of transactions on the platform.