Tjostolv Moland, one of two Norwegian men jailed in the Democratic Republic of the Congo on murder, espionage and conspiracy charges, has been found dead in his cell, his lawyer and officials said.
The cause of death was not immediately clear, attorney Hans Marius Graasvold said on Sunday.
British-Norwegian citizen Joshua French, who was imprisoned alongside Moland in 2009, is upset but in good health, Graasvold said.
The two former Norwegian soldiers were sentenced to death in Congo for the murder of their driver, Abedi Kasongo, and the attempted murder of a witness. The court in the northeastern city of Kinsangani also convicted them of spying for Norway because they were carrying military ID cards at the time.
They denied the charges and the Norwegian government has denied that they were spies.
Lambert Mende, Congo's information minister, said the Congolese government is investigating Moland's death.
"We're trying to determine whether it was suicide or homicide. It looks like suicide but we're not sure," he told the Reuters news agency, without giving further details.
Graasvold told Norwegian newspaper, Aftenposten, that Moland had been ill several times since 2009 and has been treated for malaria and psychosis. He said the two were in Congo in 2009 to research setting up an event company that would arrange extreme tourism holidays.
They had hired Kasongo as their driver after their motorbike broke down while traveling through Congo's jungle. They have always insisted that they were then ambushed by gunmen.
Norwegian Foreign Minister Espen Barth Eide said Moland was found on Sunday morning and that members of his family in Norway have been informed.
"We have been working with Congolese authorities through various channels over the years, some public, others not, without any success. I can honestly say that no case in the ministry has demanded more of our resources and attention," Eide told a news conference in Norway.
Norwegian authorities have tried to have the two transferred to serve their sentences in Norway and have also worked closely with Britain to put pressure on the Congolese officials to pardon the two.
"Our main focus now is to get French home. ... That's more important than ever," Moland added.
Moland's death was big news in Norway and dominated the headlines, relegating the upcoming elections to second place.