UPDATE: German prosecutors said the man who was first arrested in relation to the Berlin truck attack was released due to insufficient evidence, the Associated Press reported.

Following of a 23-year-old male suspect in the attack on a Christmas market in Berlin, German chief prosecutor Peter Frank has said the man in custody may not have been involved in the incident.

"We have to think that the person who was arrested yesterday, a man of Pakistani nationality, we have to be open to the idea that he could possibly not have been the attacker," Frank said during a press conference on 20 December afternoon.

The suspect was denying any involvement in the tragedy, the German interior minister Karl Ernst Thomas de Maizière said at an earlier press conference.

De Maizière confirmed the police arrested a 23-year-old man, whom is thought to have come from Pakistan. According to the minister, the suspect entered Germany on 31 December 2015, and was registered by the authorities in Berlin in February 2016.

Pakistan's Ambassador to Germany Jauhar Saleem told local media that German authorities had not contacted the embassy over the involvement of a Pakistani national.

The suspect was first identified as Naved (Navid) B. in German media, but later reports quoting security sources say that the person arrested may not have had any involvement in the attack.

"We have the wrong man," an unnamed police source told Welt newspaper. "This means the situation is different. The real culprit is still armed and can commit further atrocities."

Minister de Maizière said: "Investigations are underway, everything is open. We are looking at who is responsible for this attack."

"I will not rest until the perpetrator or perpetrators are found and are punished," he added.

The minister specified there was no longer any doubt at all that the tragedy was a terrorist attack. "The truck was deliberately driven into the crowd. This was a brutal terror attack," he said.

Berlin, Christmas Market Attack

Only a few of the victims have been identified so far, the minister said, without giving any further detail as to the identity of the 12 casualties. He said that out of the 48 people who suffered injuries, 18 are in critical conditions.

The president of the Federal Police Office (BKA) Holger Münch said at a press conference following that of the interior minister that six of the victims were confirmed to be German nationals.

One man was found dead in the passenger seat of the Polish truck that was used to plough into the crowd. The injuries on the man's body seem to have come from a gun, minister de Maizière confirmed. He said that police were looking for the gun.

Berlin truck crash
The scene where a truck ploughed into a crowded Christmas market in Berlin Fabrizio Bensch/Reuters

Addressing the nation earlier on 20 December, German Chancellor Angela Merkel promised the person responsible for the attack would be prosecuted with "all the means possible", adding that it would be "repugnant" if it was confirmed that the attacker had received protection in Germany as a refugee.

She said: "It would be very difficult for us to learn that a human being committed this deed who came to Germany to ask for refugee [status] and asylum. It would be terrible for all of the Germans who are very active day by day in helping asylum seekers and refugees. It would be repugnant."

Minister de Maizière was adamant that, while Christmas markets in Berlin will be shut down for a day of mourning, other Christmas markets and events should proceed as planned. "It is terribly important to not allow ourselves to be robbed of our lifestyle based on freedom and responsibility," he said.

He invited people to stay alert and take care of one another. He said: "It is my wish that this attitude of looking out for one another, taking care of one another is the way we live in this country. Let us stand together."