German Chancellor Angela Merkel promises that the perpetrator will be punished.
Investigators said they suspect the driver of a truck that ploughed into a crowd at a Berlin Christmas market, killing 12 people and injuring 48, did so in a deliberate terrorist attack.
"There is much we still do not know with sufficient certainty but we must, as things stand now, assume it was a terrorist attack," German Chancellor Angela Merkel said. She also said the suspect may have been an asylum-seeker.
One person in the truck died after it crashed into a crowd of people at the market, but police took another suspect into custody.
The Berlin police chief said on Tuesday it was not clear if the Pakistani man arrested for the rampage was the driver of the vehicle.
"We have the wrong man," Die Welt newspaper quoted the police chief as saying, "And therefore a new situation. The true perpetrator is still armed, at large and can cause fresh damage."
Berlin Police said on Twitter, "The temporarily arrested suspect denies the offence. Therefore we are particularly alert. Please be also alert."
The temporary arrested suspect denies the offense.— PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016
Therefore we are particulary alert.
Please be also alert.#Breitscheidplatz
"I know it would be especially hard for us all to bear if it were confirmed that the person who committed this act was someone who sought protection and asylum," she said.
Germany's chief prosecutor Peter Frank says we will know by late afternoon or evening if the arrested man is perpetrator of attack. He further said, "We do not know yet if the attack had an Islamist background."
While no one had claimed responsibility for the attack, Frank said the sequence of events evoked memories of a similar attack in the French city of Nice in July.
If a migrant link is confirmed, it could further sour sentiment towards asylum-seekers in Germany, where more than a million people fleeing conflict and poverty in the Middle East, Africa, and elsewhere have arrived this year and last.