Finnish police have started a preliminary investigation into Voislav Torden, a suspected mercenary with the neo-Nazi Rusich group in Ukraine.
Prosecutors say Torden is suspected of crimes against captured or wounded Ukrainian soldiers that took place in eastern Ukraine before Russia's full-scale assault on the country began in February 2022.
The Russian citizen cannot be extradited to Ukraine for human rights reasons, according to Finland's Supreme Court, so Finland is investigating suspected offences instead.
These are so-called "international crimes", which took place abroad but can be prosecuted under Finnish law based on international treaties the country has signed. The National Bureau of Investigation is taking the lead on the case.
He had been detained by Finnish border guards in the summer after entering and trying to leave the country on a fake passport.
He is believed to be a leader of the neo-Nazi Rusich group, which gained a reputation in 2014-15 for its cruelty in eastern Ukraine. In 2015 some of the group travelled to Syria to fight with the Wagner mercenary group.
Since the 2022 assault on Ukraine Rusich has returned to the country, fighting with Russian forces.
Ukraine had requested his extradition, but the Supreme Court ruled that conditions in Ukrainian prisons did not meet minimum standards, and he could not be handed over.
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