JUDY WOODRUFF: The Islamic State group has claimed responsibility tonight for the Berlin truck attack that killed a dozen people and injured 50.

That word came hours after German police let their main suspect go.

Special correspondent Malcolm Brabant reports from Berlin.

MALCOLM BRABANT: This normally bustling Christmas market was eerily quiet today, as investigators searched for clues.

Swathed in fog, and with armed guards sealing off the area, Berlin was coming to terms with its new status as a victim of terrorism, after Paris, Brussels and Nice.

German Chancellor Angela Merkel spoke at a morning news conference.

ANGELA MERKEL, Chancellor, Germany (through translator): There is still a lot that we don't know about this act with sufficient certainty, but we must, as things stand, assume it was a terrorist attack.

MALCOLM BRABANT: Police detained a Pakistani asylum-seeker shortly after the truck rammed into a crowd of people on Monday. But, today, he was released, due to insufficient evidence. Prosecutors said he matched a description of the suspected attacker, but he denied any involvement.

The truck, which had been carrying steel beams, was towed away earlier this morning. The body of a Polish truck driver was found inside the cabin. He had been stabbed and shot after being hijacked. The gun used to kill him has yet to be found.

PROF. PETER NEUMANN, King's College, London: It was waiting to happen, no? I'm not, like, totally surprised.

MALCOLM BRABANT: Berlin is the hometown of terrorism expert Peter Neumann. He believes German authorities were too complacent about the possibility of an attack and didn't offer sufficient protection to the Christmas markets.

PROF. PETER NEUMANN: I think, in Germany, people have been very blessed with the idea that they would be spared this kind of attack. And so I don't think that German authorities were thinking as systematically about the threat from terrorism as authorities in Britain have or authorities in other countries.

This will have to change. And there will be a very uncomfortable discussion in Germany about anti-terrorism measures, but also, of course, about the relationship with Muslim communities and with refugees.

MALCOLM BRABANT: The chancellor and other senior officials laid white roses at a makeshift memorial to the victims. They toured the market and were briefed by senior police officers.

Ordinary Berliners paid their respects, laying candles, flowers and messages of sympathy. Many said they believe this attack marked a turning point for Germany, especially as, for most of the past 24 hours, they were under the impression that the massacre had been perpetrated by an asylum-seeker admitted under Chancellor Merkel's open-door refugee policy.

SABINA AGARUNOVA: I definitely want to see stricter border control in Germany and in the European Union. I think I want to see more cooperation between the countries of the European Union and regarding refugee policy, because, at the moment, it's out of control.

HERMANN BORGHORST, Former Social Democrat Politician: I think it's not yet today the time to think about the consequences for the refugee politics. I think today it's a question of sadness and come here, see people come together, and bring flowers. And that's most important. And I think we have a good system of asylum-seekers.

MALCOLM BRABANT: Chancellor Merkel warned against letting the attack change Germans' way of life, but she acknowledged her policies were under new pressure.

ANGELA MERKEL (through translator): I know that it would be particularly hard for us all to bear it if it were to be confirmed that a person committed this crime who sought protection and asylum in Germany. This would be particularly sickening for the many, many Germans who work to help refugees every day and for the many people who really need our help and are making an effort to integrate in our country.

MALCOLM BRABANT: At the Berlin State Parliament, where flags were lowered to half-mast, the anti-immigrant Alternative for Germany Party and its regional deputy leader, Ronald Glaser, offered a harsher appraisal.

RONALD GLASER, Regional Deputy Leader, Alternative for Germany Party: First of all, the terrorist is to blame for what happened. On the other hand, Angela Merkel and her welcome culture made all these refugees come to our country. And among them were a lot of fanatics and criminals. She's not going to accept that her policy was wrong, but, of course, the German people is awakening and they maybe will force her to do so.

MALCOLM BRABANT: Just outside the regional parliament is the largest remaining section of the Berlin Wall, whose demise 27 years ago led to Europe's open borders. There could not be a more poignant reminder of what's at stake as Europe grapples with terrorism and the wave of migration across the Mediterranean.

In the meantime, the German capital remains on high alert.

KLAUS KANDT, Police Chief, Berlin (through translator): Of course people are worried. I believe people who live in the city should be vigilant.

MALCOLM BRABANT: Security has also been tightened at Christmas markets and other outdoor venues all across Europe, out of an abundance of caution.

And in Berlin this evening, mourners gathered for a candlelight vigil in the same square where the tragedy took place, as Germany's leaders attended a solemn memorial service at a nearby church. The iconic Brandenburg Gate was illuminated with the images of both the German and Berlin flags.

JUDY WOODRUFF: And Malcolm joins me now from Berlin.

Malcolm, now that ISIS has claimed responsibility, how is that affecting thinking there?

MALCOLM BRABANT: It's not really made that much of an impact, to be honest.

The German interior minister acknowledged that ISIS had made this claim, but he didn't really react to it. I don't think anybody is particularly surprised. This attack was right out of the jihadist playbook. It's the sort of language in the claim that they have made that they have used before.

But what, I think, ISIS has achieved by saying what they have done is that they have perhaps helped to polarize Germany. They have helped to perhaps drive people into the arms of the Alternative for Germany Party, the right-wing anti-immigrant party. And they're going to create hatred. And that's really what their main objective is.

JUDY WOODRUFF: So, the perpetrator is still on the loose. Is there fear that he or she may strike again?

MALCOLM BRABANT: That certainly is the worry.

And people are being advised to stay indoors. The police are very much concerned. The interior minister did say that they do have some leads and that the police are following up on those leads.

But they thought that they actually had their man in their sights, because the person they described as a brave witness to the attacks said that he followed the person that he thought was the truck driver. He had driven through the Christmas market, and was in touch with police all the time.

But it now appears that he may have actually lost sight of the target during that particular time, which is perhaps why the wrong man was arrested.

But there are certainly fears that there will be attacks again, and there are armed police at other Christmas markets around there. One pretty good indication of just how tense things are, there was a federal prosecutor who was giving a press conference in Germany earlier today, and he said that he certainly wouldn't go to any Christmas markets, which is hardly an endorsement of the state of security here.

JUDY WOODRUFF: No, it isn't.

Malcolm Brabant in Berlin, thank you.