German prison
A neo-Nazi network is believed to have spread throughout prisons in the German state of Hesse. REUTERS

German authorities have begun to uncover an underground network of neo-Nazis operating throughout numerous prisons.

While the full extent of the network remains unknown, it has affiliations with the National Socialist Underground, a far-right terrorist group.

Officials in the German state of Hesse, where the criminal network is said to have originated, confirmed that they have opened an investigation.

"We know that far-right criminals are trying to build up networks and new organizational structures from prisons,” said Hesse’s justice minister Joerg-Uwe Hahn, Reuters reported. “We will stop this.”

Prison guards discovered inmates suspected of being part of the network were communicating through codes hidden in magazines and letters.

They also found the names of National Socialist Underground contacts inside inmates’ cells, including one of the group’s alleged founders Beate Zschaepe, who's set to go to trial next week along with four other suspected National Socialist Underground members in connection with at least 10 racially-motivated killings between 2000 and 2007.

Hahn referenced previous criticisms of German authorities for not responding adequately to the threat of the National Socialist Underground.

"We don't want to repeat the mistakes made by security authorities in relation to the crimes of the National Socialist Underground,” Hahn said.

The neo-Nazi prison network also has connections with another far-right group known as the HNG, an aid organization that specifically provided financial support to imprisoned neo-Nazis and their families.

In 2011, the German government banned the HNG, which Hahn believes led to the formation of a new network within the prison system.

“You have to realize that the NSU was just the tip of the iceberg,” said Bernd Wagner, a former police detective and now Berlin-based security consultant on the far right, Deutsche Welle reported.

“You have to realize that there are several overlapping, linked-up networks. It's a whole system of networks and relationships," he added.