German police are hunting for the perpetrator of Monday's truck attack at a Berlin market which killed 12.

Germany's Spiegel Online reports that investigators are looking for a Tunisian after an ID card was found in the truck. Police have not commented.

Officials released the only detained suspect on Tuesday, saying there was no evidence to link him to the attack.

They now say more than one suspect may be on the run, possibly armed, and security has been stepped up.

So-called Islamic State (IS) said one of its militants carried out the attack, but offered no evidence.

The former suspect, a Pakistani national named as Naved B, had denied any connection with the attack.

Police said there were no forensic clues to link him to the vehicle, and had expressed doubt about his involvement soon after his arrest.

Early on Wednesday police said another suspect was arrested overnight but later released, according to broadcaster RBB.

Spiegel reported that police were looking for a Tunisian named Anis A, born in 1992 in the city of Tataouine, after a temporary-stay permit was found under a seat in the cab of the lorry.

The report said the suspect is known to use two aliases, but did not give details. Police have not confirmed the report.

German media also reported that a police operation was under way in the state of North Rhine-Westphalia, but there are no details so far.

Meanwhile, it has emerged that the original driver of the truck may have fought the attacker as the vehicle was being driven into the market.

Polish citizen Lukasz Urban was found dead on the passenger seat with gunshot and stab wounds.

Investigators quoted by German media say there is evidence that, despite being stabbed, Mr Urban wrestled him for the steering wheel.

One official quoted by Bild newspaper said it appeared from the post-mortem examination that the driver had survived up to the attack and was shot dead when the truck came to a halt. No gun has been recovered.

Ariel Zurawski, the owner of the Polish transport company, said he had been asked to identify Mr Urban from photographs.

"His face was swollen and bloodied. It was really clear that he was fighting for his life," he told broadcaster TVN.

Company manager Lukasz Wasik described Mr Urban as a "good, quiet and honest person" and said he believed he would have defended the truck "to the end".

'Be vigilant'

Police say they are acting on hundreds of tips from the public and are examining DNA traces from the cab of the truck.

"I am fairly confident that we will have a new suspect tomorrow or very soon," Andre Schulz, head of the BDK police union, told broadcaster ZDF late on Tuesday.

Berlin Mayor Michael Mueller said security had been increased to prevent copycat attacks.

"The right thing is to keep your eyes open - be vigilant in this situation. But you can move around safely in public places," he said.

IS claimed the attack through its self-styled Amaq news agency, saying it was "in response to calls to target nationals of the coalition countries".

Prosecutor Peter Frank told reporters that the style of attack and the choice of target suggested Islamic extremism.

But Mr de Maiziere reacted cautiously to the claim, saying "several lines of investigation" were being pursued.

The attack took place at the Breitscheidplatz Christmas market in Berlin, where the lorry was driven through the crowd and several stalls for a distance of 80m (87 yards) .

In addition to the 12 dead, 49 people were injured. At least a dozen remain in critical condition.

Monday's incident mirrored the lorry attack on Bastille Day crowds in the French city of Nice on 14 July, which was also claimed by IS.

Both IS and al-Qaeda have urged their followers to use vehicles as a means to attack crowds.

A vigil was held on Tuesday evening at the Kaiser Wilhelm Memorial Church, next to the scene of the tragedy.

German Chancellor Angela Merkel, whose "open door" policy on migrants has been criticised by her opponents, acknowledged fears that the attacker could be one of them.

"I know that it would be particularly difficult for us all to bear if it turned out that the person who committed this act was someone who had sought protection and asylum in Germany," she said.

But she vowed to punish those responsible "as harshly as the law requires".

Her opponents, including the far-right Alternative fur Deutschland (AfD) said she had compromised Germany's security by letting migrants in without sufficient checks.