(CNN)Norwegian multimillionaire Tom Hagen has been arrested on suspicion of killing his wife, Anne-Elisabeth Hagen, who disappeared 18 months ago.
Hagen's lawyer denies the property and energy mogul was involved, and he has not yet been charged with any crimes.
Anne-Elisabeth Hagen, 69, disappeared in October 2018 and the family said a ransom had been demanded for her release, but on Tuesday Norwegian police arrested Hagen, 70, on suspicion of killing her, according to public broadcaster NRK.
The case was first reported in January 2019, attracting widespread attention in a nation where crime rates are low.
Police say they now believe Anne-Elisabeth was killed and not abducted, and have enough evidence to "suspect" Tom of her killing.
"It is important to emphasize that although we have charged Tom Hagen, the case is still being investigated and there are several unanswered questions," Øst Police District said in a statement.
"It will be particularly important to clarify the role played by Tom Hagen, to find Anne-Elisabeth Hagen and to determine whether other people are involved."
"As the case initially appeared, our main theory was that Anne-Elisabeth Hagen had been abducted by someone with a financial motive. And in June 2019, we came to believe that she had most likely been killed," police said.
"We now believe there was no abduction and there was never any genuine negotiations. In other words there was a clear and well-planned attempt at misleading the police," the statement added.
The Norwegian legal system allows police to hold an individual on suspicion of a crime before formal charges are brought.
Hagen's attorney, Svein Holden, said his client denies the accusations.
"He strongly maintains that he has nothing to do with this," he told a group of reporters, according to Reuters.
Hagen will appear in court on Wednesday, the news agency reports.
Police asked the court to remand Hagen in custody for four weeks, with a ban on visits and outside communication, according to the statement.
They initially kept Anne-Elisabeth's disappearance a secret in a bid to protect her but then made the case public in January 2019.
"We can confirm that there is a ransom, and that serious threats have been issued," Øystein Stavdal Paulsen, senior adviser of Norway's East Police District, told CNN at the time.
"Regarding the ransom, we do not want to confirm what amount or in what form," he added.
NRK reported at the time that multiple messages were found inside the Hagens' home and that a ransom had been demanded in the cryptocurrency Monero.
Hagen, a real estate investor and power facilities owner, is one of Norway's richest men, with a net worth of around $200 million, according to Norwegian financial magazine Kapital.
Anne-Elisabeth Hagen was a board member of her husband's holding company until September, according to NRK.
The couple lived in Fjellhamar, a village about 12 miles northeast of Oslo, according to Norwegian public broadcaster NRK. The couple married in 1979 and have three adult children, it said.