German authorities are following a new lead in the hunt for the person responsible for the attack on a Christmas market in Berlin, which left 12 people dead.

According to the German newspaper Spiegel, investigators found a form of identification document underneath the driver's seat in the truck used to drive into the market: a notice of temporary stay/asylum. The document was issued to a Tunisian citizen named Anis A, born in 1992 in the city of Tataouine.

The police is now planning an "imminent" operation in the western state of North Rhine-Westphalia related to the attack, German news agency reported.

"I am relatively confident that we can present a new suspect as early as tomorrow or in the near future," Andre Schulz of the BDK union said, to TV station ZDF, on 20 December, adding that there was "good evidence" and "a lot of leads."

The police launched a manhunt for the attacker, after releasing a Pakistani man on 20 December – who was first arrested in connection with the attack – as there was insufficient evidence to hold him.

Speaking at a press conference on 20 December, German Interior Minister Karl Ernst Thomas de Maizière said there was no longer doubt that the tragedy was a terrorist attack. "The truck was deliberately driven into the crowd. This was a brutal terror attack," he said.

The Islamic State claimed responsibility for the attack through its news channel Amaq, but there is no evidence as yet for its claim.

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General view of the scene where a truck ploughed into a crowded Christmas market in Berlin Pawel Kopczynski/Reuters

Addressing the nation earlier on 20 December, German Chancellor Angela Merkel said it would be "repugnant" if it was confirmed that the attacker had received protection in Germany as a refugee.

She said: "It would be very difficult for us to learn that a human being committed this deed who came to Germany to ask for refugee [status] and asylum. It would be terrible for all of the Germans who are very active day by day in helping asylum seekers and refugees. It would be repugnant."

Merkel asked Germans to be resilient in the face of terror: "We do not want to allow ourselves to be paralysed by terror," she said. "We will find the strength to live the life as we want to live in Germany. In freedom, in openness and together."

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L-R) Mayor of Berlin, Michael Muller, German Chancellor Angela Merkel, German Interior Minister Thomas de Maiziere and Interior Senator, Andreas Geisel lay flowers near where yesterday a lorry ploughed through a Christmas market on 20 December 2016 in Berlin, Germany Sean Gallup/Getty Images

Merkel and other German officials paid tributes to the victims on 20 December, visiting the Breitscheidplatz market square and laying flowers in front of the Kaiser Wilhelm Memorial Church.

Lukasz Urban, a Polish driver, was found in the passenger seat of the truck. Investigators think he was shot dead, but they have yet to locate the gun, or ascertain at which point the man was killed during the tragedy.

According to Ariel Zurawski, the victim's cousin and employer, Urban must have been fighting for his life. "His face was swollen and bloodied. It was really clear that he was fighting for his life," Zurawski told the Polish network TVN, after identifying the body through photographs shown to him by the Berlin police.

Following Maizière comments, at the 20 December press conference, president of the Federal Police Office (BKA) Holger Münch said that six of the victims were confirmed to be German nationals. Among the missing are an Israeli woman visiting Berlin and an Italian woman who lived in the city, Israeli and Italian media reported.

The BKA has announced a press conference at 13.00 local time (12.00 GMT).