* Tunisian suspect on the run
* Four arrests in Dortmund, media say
* Merkel hopes perpetrator will be caught soon
* Attack raises security fears elsewhere in Europe
Fingerprints from a Tunisian migrant have been found inside the truck that smashed through a Berlin Christmas market on Monday in an attack that killed 12 people, German Interior Minister Thomas de Maiziere said.
A hunt is under way across Europe for Anis Amri, 24, as Germany reels from the worst attack on its soil since 1980.
"We can report today that we have new information that the suspect is with high probability really the perpetrator," de Maiziere told reporters on Thursday.
"In the cab, in the driving cabin, fingerprints were found and there is additional evidence that supports this."
Chancellor Angela Merkel, appearing alongside de Maiziere at the federal police office, said she hoped the perpetrator would be arrested soon.
Islamic State has claimed responsibility for the attack, in which the truck mowed through a crowd of people and bulldozed wooden huts selling Christmas gifts and snacks beside a famous church in west Berlin.
One of the victims was the Polish driver from whom the truck had been hijacked. His body, stabbed and shot, was found in the cab.
Amri had been identified by security authorities as a potential threat and rejected for asylum, but authorities had not managed to deport him because of missing identity documents.
The suspect involvement of a migrant - one of more than a million allowed into Germany in the past two years - has intensified political pressure on Merkel, who plans to seek a fourth term in elections next year.
Armin Schuster of her Christian Democratic party told broadcaster NDR: "We need to send the signal: Only set off for Germany if you have a reason for asylum."
The federal prosecutor's office said it would give a statement at 6 pm on the state of the investigation.
Ringed by concrete bollards, the Berlin market reopened on Thursday, with candles, flowers and flags laid amid the small festive huts in tribute to those killed.
Police in the western city of Dortmund arrested four people who had been in contact with Amri, media reports said, but a spokesman for the chief federal prosecutor denied that and said he would give no further details on the operation.
Bild newspaper cited an anti-terrorism investigator as saying it was clear last spring that Amri was looking for accomplices for an attack and was interested in weapons.
The paper said preliminary proceedings had been opened against him in March based on information he was planning a robbery to get money to buy automatic weapons and "possibly carry out an attack".
In mid-2016, he spoke to two Islamic State fighters and Tunisian authorities listened in on their conversation before informing German authorities. Amri also offered himself as a suicide attacker on known Islamist chat sites, Bild said.
Police started looking for him after finding an identity document under the driver's seat of the truck. Broadcaster rbb said the perpetrator lost both his wallet and mobile phone while running away from the attack site.
Klaus Bouillon, head of the group of interior ministers from Germany's federal states, said militant Islamists often left behind identity documents - as had also happened in France - to steer public opinion against refugees, further inflaming tension.
The attack has heightened security concerns across Europe in the approach to Christmas. In France, target of three major attacks in the last two years, security around festive markets was strengthened with concrete barriers, and troops were posted at some churches.
The Italian Foreign Ministry said an Italian woman named Fabrizia Di Lorenzo was among the Berlin victims, and the Israeli Foreign Ministry said an Israeli woman called Dalia Elyakim had been identified among the dead.
Candles, flowers and messages are placed at a makeshift memorial for the victims of the Christmas market attack in Berlin on Thursday.