His warning came as a police union, the Federation of German Criminal Police (BDK), suggested that an alarm button be set up for the internet so that web users could report extremist content or threats quickly. It is one of a growing number of calls for greater scrutiny of web-based extremism.
Friedrich told the Wednesday edition of the daily Rheinische Post that although far-right organisations appeared to be suffering membership losses, the number of violent, unattached nationalists and neo-Nazis was climbing.
Security services were watching the far-right scene closely, he said. Yet Friedrich added that he could never rule out an attack in Germany of the kind that has shaken Norway because there were always extremists who lurked in the shadows.
Avowed Norwegian anti-immigration nationalist, Anders Behring Breivik, killed at least 76 people in a shooting rampage and a bomb attack last Friday. Most of his victims were young members of the ruling Labour Party attending an island summer camp.
“Even if we monitor the scene intensely, we cannot rule out that lone, self-radicalised individuals go unnoticed,” Friedrich said. “We know there is some danger in the right-wing extremist scene. But the problem is not those who we have in our sights but those who radicalise themselves in secrecy.”
Breivik appears to have been active on the internet and claims to have had contact with right-wing extremists in other countries. However he was not an active or well-known member of Norway's far-right and went unnoticed by security services, save for his purchase of large quantities of fertiliser to make explosives - purchases he covered by renting a farm.
BDK chairman Klaus Jansen told the Neue Osnabrücker Zeitung that an alarm button for the internet would enable people who came across dangerous content to report it quickly.
“People who discover right-wing radical content, Islamist ideas or indications of a mass murder have to be able to freeze the page and transmit (the information) to an emergency centre,” he said.
The emergency centre would need to be “staffed around the clock by specially trained police, sociologists or psychologists,” he said.
The Norwegian killer, Jansen pointed out, had spread his ideology on the internet via Facebook and other sites.
“Something was concocted there that many people on the internet were aware of.”
Greens co-leader Cem Özdemir also criticised the lack of accountability on the internet and said it should not be exempt from the laws of the constitution that applied to other areas of life.
“The internet is not a punishment-free zone,” he said. “There are definite problems with servers that are in other countries. There are problems where national borders are crossed. In these cases, we need to do more to ensure security.”
Friedrich said he was also concerned about so called “free nationalists,” who do not belong to any far-right party or organisation and therefore are less predictable and harder to observe. They are thought to number at least 1,000 in Germany and are considered to be more violence-prone.
Reports from the Office for the Protection of the Constitution (BfV) show they are rising in number and of greater concern than traditional, organised right-wing extremists.
Free nationalists follow the model of left-wing extremists, so-called Linksautonomen, who are shadowy, less predictable and less observable, the BfV has said.