German authorities say they are tightening security measures after discovering a neo-Nazi network inside the country's prisons.

The network, which was used to try and organize financial help for far-right convicts and their relatives, was discovered in the state of Hesse after evidence was uncovered during a search of prison cells.

Authorities say coded messages were sent between prisons by letter through the mail and also hidden in magazines.

Hesse state Justice Minister Joerg-Uwe Hahn told Bild newspaper Wednesday his office is tightening controls to prevent any new networks from developing.

Hahn's office confirmed reports the network attempted to contact people connected to a small neo-Nazi group that killed 10 people but would give no specific details. The group's sole surviving member goes on trial next week.