Germany Chancellor Angela Merkel has said Tuesday's Berlin Christmas market incident that saw a lorry crash into a crowd killing at least 12 people, was a "terrorist act".
Angela Merkel said on Tuesday, a day after the event in the German capital, that it was "a very difficult day for Germany".
"We have to assume that this was a terrorist attack," Merkel said.
She said that it would be "particularly sickening" if the attacker was in fact a refugee.
Police had detained a 23-year-old Pakistani asylum seeker. However, late on Tuesday, he was released because of insufficient evidence, German prosecutors said.
Germany's top prosecutor, Peter Frank, said that while investigators were treating the attack as an act of "terrorism", though there is no claim of responsibility yet.
Frank also told reporters that it was not entirely clear whether there was one perpetrator or more.
READ MORE: Lorry ploughs into Christmas market crowd in Berlin
The lorry struck the popular Christmas market outside the Kaiser Wilhelm Memorial Church late on Monday as tourists and locals enjoyed a traditional pre-Christmas evening near Berlin's Zoo station. Dozens of people were wounded.
The interior ministry said Christmas markets in Berlin will remain closed on Tuesday out of respect for the victims.
A damaged windscreen of a lorry that slammed into a crowd at a Christmas market on Breitscheidplatz square near the fashionable Kurfuerstendamm avenue [Reuters]
Witness Jan Hollitzer told Al Jazeera the truck drove about 50 metres through the market.
Recalling the scene, Hollitzer said the vehicle was travelling "very fast".
"It was really horrible. There were many casualties and injured people," he said. "I saw people hit by the truck and also people under the truck ... I can tell you those are images you don't want to see."
Among the dead was a man in the lorry, who died as paramedics treated him, Berlin police spokesman Winfried Wenzel said. Police identified him as a Polish national, but didn't give further details.
The lorry had a Polish licence plate, police said. The Polish owner of the truck said he feared the vehicle may have been hijacked.
A tow lorry operates at the scene where a truck ploughed through a crowd at a Christmas market [Fabrizio Bensch/Reuters]
The interior minister of the German state of Saarland said on Tuesday that Germany is in a state of war.
"We must say that we are in a state of war although some people, who always only want to see good, do not want to see this," Klaus Bouillon told German broadcaster SR.
The incident came less than a month after the US State Department called for caution in markets and other public places across Europe, saying armed groups such as the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) and al-Qaeda were focusing "on the upcoming holiday season and associated events".
ISIL and al-Qaeda have both called on followers to use lorries in particular to attack crowds
On July 14, a lorry ploughed into Bastille Day revellers in the southern French city of Nice, killing 86 people. ISIL claimed responsibility for that attack, which was carried out by a Tunisian living in France.
|Flowers and candles are placed near the Kaiser Wilhelm Memorial Church in Berlin after a lorry ran into a crowded Christmas market [Britta Pedersen/AP]
Germany has not experienced any mass-casualty attacks by armed groups, but has been increasingly wary since two attacks by asylum seekers in the summer were claimed by ISIL.
Five people were wounded in an axe attack on a train near Wuerzburg and 15 in a bombing outside a bar in Ansbach, both in the southern state of Bavaria. Both attackers were killed.
Those attacks, and two others unrelated to armed groups in the same week-long period, contributed to tensions in Germany over the arrival last year of 890,000 refugees.
The far-right party AfD, which has been critical of Merkel's immigration policy, has gained support in recent regional elections, as it blames Europe's security woes on refugees.
Horst Seehofer, leader of Bavaria's Christian Social Union - the sister party to Merkel's Christian Democrats - called for a review of Germany's security policies.
"We owe it to the victims, to those affected and to the whole population to rethink our immigration and security policy - and to change it," said Seehofer.
Source: News agencies