The National Bureau of Investigation (NBI) and police in Häme suspect four members of a far-right extremist group of committing an aggravated firearms offence with terrorist intent.
Three of the suspects are also believed to have trained for committing a terrorist offence.
The group also held staunch anti-immigrant views, police said.
During the course of a preliminary investigation, officers learned that the Lahti-based group used a 3D printer to manufacture FGC-9 submachine guns.
Investigators believe the suspects were influenced and motivated by accelerationist ideology, which refers to using violence to expedite the collapse of democratic society, a school of thought which far-right movements have embraced in recent years.
Read more: YLE (Finland)