The suspect behind the Berlin lorry attack is on the the run after the man arrested by police was released due to insufficient evidence.
A 23-year-old Pakistani asylum seeker was detained in the aftermath of Monday's attack but denied any involvement. German police have admitted the person behind it may still be armed and at large.
Federal public prosecutor Peter Frank said the "modus operandi" of the attack had echoes of July's atrocity in Nice, in which 86 people died, and could have been the work of Islamic extremist groups, with the target of the attack "highly symbolic".
On Tuesday night Islamic State claimed responsibility for the attack that killed 12 people and injured 50.
Eleven people died when the articulated lorry careered through huts and stalls at the Christmas market near Kaiser Wilhelm Memorial Church, along with a Polish man found shot dead inside the lorry's cab.
Six of the 12 victims have been identified as German.
Holger Munch, head of the federal criminal police office, said the attack marked the realisation of a threat authorities were already aware of, adding that the country is now on a state of "high alert".
German chancellor Angela Merkel and senior officials visited the scene of the attack on Tuesday, laying white roses among candles at a makeshift shrine to the dead and injured.
Earlier Mrs Merkel said: "Millions of people, including myself, are asking ourselves, how can you live with the fact that, while celebrating the festive season where we want to celebrate life, somebody has come along and took so many lives? I only know that we do not want, and we cannot live with it.
"We do not allow ourselves to be paralysed by terror. Although this might be difficult in these hours, but we will find a strength to continue living life as we want to live it in Germany, in freedom and openness and together."
Prime Minister Theresa May paid her own tribute, saying events in Berlin had "shocked us all" as she offered condolences to those affected.
The attack has led to heightened security concerns in the UK, though a Number 10 spokesman said there were "no plans" to change the UK's security level, which currently stands at "severe" - meaning a terror attack is highly likely.
But police, including Scotland Yard, are reviewing preparations already in place to protect public events over the Christmas and New Year period.
The Met announced that it would it be bringing forward enhanced security measures for Changing the Guard at Buckingham Palace, and implementing road closures during the ceremony.
Greater Manchester Police also said it had strengthened its presence at Christmas markets, which have almost 350 stalls spread across 10 sites in the city.