Via DW:

But for the last 20 years, Germany has become increasingly aware that it harbors an underground network of neo-Nazis that assassinates politicians, stockpiles weapons, compiles lists of the addresses of its political opponents, and plants nail-bombs in immigrant communities.
And a series of scandals indicate that that network has supporters in the police, the army, and the domestic intelligence agencies.
The most spectacular evidence of this came to light in 2011 when it emerged that three terrorists calling themselves the National Socialist Underground (NSU) had committed 10 murders and several bombings over the previous decade without ever being caught, or even officially identified.