Three of the men, identified only as Florian L. and Oliver R., both 35, and 36-year-old Axel H., are suspected of belonging to an extreme left organization calling itself mg, or «militant group,» prosecutors said in a statement.
The men were caught attempting to set fire to a truck owned by the German army in the eastern state of Brandenburg early Tuesday, prosecutors said.
A fourth man, Andrej H. 36, was also arrested on suspicion of supporting the group, which prosecutors said was founded in 2001 and is known to authorities for a number of attacks, beginning with setting fire to a vehicle at a Berlin car dealership in that year.
Federal authorities say mg is also suspected of sending threatening letters containing live cartridges to several German officials.
Federal prosecutors, based in Karlsruhe, are investigating the group and its activities. No formal charges have been brought. If tried and convicted of belonging to a terrorist organization, the men could face a maximum sentence of 10 years in prison.