According to a press release from the prosecutor, they are a "right-wing terrorist" association formed in September last year.
One of the twelve people arrested is a police officer, reports the AFP news agency citing a source within the Interior Ministry in the state of North Rhine-Westphalia. The police must have previously been suspended from their work due to suspicions of links to right-wing extremism.
It is unclear if the police are one of the four main suspects.
All the arrested are German citizens and, among other things, they must have communicated with each other via messaging apps.
After several acts of violence, German authorities have put more resources on the extreme right.