According to the National Bureau of Investigation (NBI) director and detective chief inspector Jari Räty, the man fought on the Iraqi side against the extremist organisation ISIS.
Räty says that he cannot at this stage comment in more detail on the war crime, but the suspected act is a lesser war crime.
The police tracked down the suspect last week. The man was caught in the Hämeenlinna area.
War crime is a very broad term that covers a range of offenses from killing and raping to misuse of the white flag of surrender.