It would not be the first time an anti-Muslim attacker has been mistaken for an Islamist extremist in Germany.

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Germany is still wrestling with the anti-Muslim terror group National Socialist Underground (NSU), which killed 10 people — most of them Turks — between 2000 and 2007. Investigators had initially blamed Germany's immigrant community for most of the deaths, characterizing them as the result of infighting and organized-gang activity.

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Two of the NSU suspects later killed themselves; a third, Beate Zschäpe, is on trial in Munich. The attacks have fostered deep mistrust between Germany's large immigrant community and authorities: The country's intelligence services stand accused of having deliberately ignored clues that right-wing extremists had carried out the killings.

Last year, authorities raided homes across Germany and took four anti-Muslim terrorism suspects into custody. They were accused of having planned attacks on mosques and asylum seekers.

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The suspects — who had recently founded a group called "Oldschool Society" — were believed to be right-wing extremists. "The four arrested procured explosives for possible terror attacks by the group," a police statement specified. According to Germany's Der Spiegel magazine, an attack involving a refugee housing center might have been imminent.

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Are German authorities "blind on their right eyes?"

Critics of the government say it is striking that the only terror group that was able to kill without being held accountable for years was not an Islamist organization but a homegrown extremist group that primarily targeted Muslims.

In recent months, new details have emerged that have prompted questions about whether Germany's negligence of the far-right threat might be even more acute than first assumed.

After revelations of the alleged NSU crimes, a review of thousands of cases suggested that 849 more people than originally thought could have been killed by right-wing German extremists since 1990. Critics say authorities had ruled out right-wing extremism as a motive for those killings, despite evidence suggesting otherwise. Whether those incidents can now be categorized as right-wing terrorism is uncertain, though.

There are concerns that tensions are on the rise. Germans, like many other Europeans, are increasingly opposed to the rising number of immigrants seeking asylum in their country.

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Other countries also have dealt with attacks on Muslims, although the extent to which they could be categorized as terrorism remains a matter of dispute. In July 2013, for instance, a nail bomb exploded in front of a mosque in Tipton in the West Midlands, England. Nobody was injured, but the BBC quoted police official Gary Cann as describing the incident as an "act of terrorism." Last year, The Washington Post reported that French Muslim leaders had counted more than 50 anti-Muslim attacks within the week after the attacks on the editorial offices of Charlie Hebdo and a Jewish supermarket in Paris.

In Germany, right-wing extremists seem to be especially well connected

The attacker on Friday is thought to have operated alone, according to German media reports.

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But in many other cases, investigators say, German right-wing extremists rely on a network that has grown over decades. German intelligence services are believed to have hired some of the extremists as double agents, creating its own set of complications.

The revelations have not significantly lowered the risk that Muslims and other immigrants might face in Germany: Last year, more than 200 asylum centers were set on fire, according to an analysis by the German weekly Die Zeit. Right-wing extremists committed most of those crimes.