They beat up black and Arab football fans, terrorise immigrant neighbourhoods, smash Muslim and Jewish gravestones, preach hate and rally support online. Norway's attacks laid bare a fringe of flourishing racist anger around Europe - and exposed the risk that it could erupt into violence anywhere, anytime.
In interviews and online forums, European far-right extremists have not softened their rhetoric since Norway's massacre left 77 dead. They may recoil at the attacker's methods, but not his message: that Muslim immigrants are a threat to European survival.
"If there were no immigrants, there would never have been this drama," one French blogger says. Jean-Marie Le Pen, France's firebrand icon of anti-immigrant politics, said Norway "did not estimate the global danger that massive immigration represents, which is the principal cause" of the attacks.
The rise of anti-immigrant thought in the European political mainstream may even be increasing the risk. As far-right parties such as Le Pen's National Front have become more moderate and appealing to a broader spectrum of voters, they have jettisoned their most extreme members - creating rage at the farthest fringes.
"These most extreme members are then left alone without the moderating influence of their former party colleagues," said Marko Papic of US-based analyst group Stratfor.
In Internet chatter, some of the most extreme European voices even say Anders Behring Breivik wasn't xenophobic enough. Spain's Democracia Nacional, Russia's Slavic Union and the Swedish Resistance Movement dismissed him as a Zionist.
Europe's right-wing extremists are exceptional voices, numbering in the thousands. But their voices can take on disproportionate weight and skew perceptions of immigration.
Foreign-born people made up 9.4% of the population of the 27 European Union states last year, or 47 million of the EU's half a billion citizens, according to statistics agency Eurostat. But millions of those "foreigners" originally came from another EU country.