BERLIN — Four attacks in 11 days have us worried that Germany is at the beginning of a campaign of attacks by individuals that will not be easy to stop.

Like the July 14 attack in Nice, France, the perpetrators in recent days in the German cities of Würzburg and Ansbach most likely did not act alone. There are, it can be presumed, accomplices, accessories, mentors.

In the case of the axe attack on the train in Würzburg last week that injured three seriously, investigations have revealed that the culprit had been extremely active online. Investigators, meanwhile, say the suicide bomber who injured 15 near a musical festival Sunday in Ansbach had repeatedly used his cell phone, shortly before the attack.

It looks as if Europe is witnessing the awakening of a series of sleeper networks. The attackers themselves are often, just like in the Middle East, either hardened criminals or mentally ill, or both. They are being piloted by religious fanatics, but have so few detectable links with them that they can fly under the radar.

Leaving traces

Neither in Nice, nor in Würzburg or Ansbach, were there any warnings from foreign services — be it from the Middle East or from Western alliance partners. This was not the case with the Sauerland group that wanted to attack the Ramstein military base in 2007, or the Bataclan attackers in Paris last November, or those that struck in Brussels in March: All attacks were carried out by groups of people familiar to law enforcement.

Individual perpetrators can count on the efficiency of keeping cover, forcing authorities to constantly search for the proverbial needle in the haystack. Still, even these individuals leave traces.

What's particularly worrying in the case of Würzburg is that the culprit's Facebook activity would have been open to surveillance.

Same thing for the person who carried out the mass shooting in Munich on Friday, who had no connection to Islamic terror groups, but whose procurement of weapons and planning activity may have raised red flags online. But it's a lot to ask of the security authorities to make the right connections between clues that may have occurred weeks or months away from each other, also often separated geographically. After the attack, such connections look rather obvious.

Still, it is not impossible to detect signs of potentially dangerous people. It's a tricky topic that may involve issues of medical confidentiality and the like. But after the last 11 days, we unfortunately have to be prepared for further such attacks — and they may be on a much bigger scale.

New questions concerning domestic security in Germany, which until now have been largely avoided, are bound to be put on the table. The value of evidence gathered from petty criminals will take on a whole new relevance in light of the background of the recent attackers.

And of course, the elephant in the room: Germany's refugee policy must be put to the test. Prohibition of deportations to countries facing civil war must continue to be the policy, but only for law-abiding refugees. If more attacks like these are coming, all asylum seekers will inevitably be forced to face broader supervision. This was indeed what the British did during World War II, and is thus not necessarily a violation of common democratic habits and customs.