Updated | German police unsure of Berlin attack suspect’s involvement

German police think that the man from Pakistan who was arrested as a suspect in the attack on a Berlin Christmas market that killed 12 people on Monday was not the actual perpetrator, Die Welt newspaper reported, citing senior security sources.

"We have the wrong man," a senior police chief said. "And therefore a new situation. The true perpetrator is still armed, at large and can cause fresh damage," the paper quoted the source as saying.

Berlin police chief Klaus Kandt later said he could not confirm the suspect in custody was the driver of the lorry.

Germany's interior minister, Thomas de Maizière, said the suspect has denied any involvement.

During a press conference, Holger Münch, head of the Federal Criminal Police Office, said that “we need to work on the assumption that an armed perpetrator is still on the loose. As a result of this we are on high alert.”

The man being held is believed to have arrived in Germany from Pakistan at the end of last year.

A suspicious person was arrested near #Breitscheidplatz. Whether it is the driver of the truck, is currently under consideration.

— PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 19, 2016

He was captured in a park 2km away after reportedly fleeing the popular Christmas market in west Berlin.

On Tuesday, police are treating the incident as a terror attack.

The truck had a runup of about 80 metres before crashing into people gathered around wooden huts serving mulled wine and sausages at the foot of the Kaiser Wilhelm memorial church, at about 8:00pm. Besides the 12 deceased, the incident also caused 48 injuries, some serious.

A sad certainty: Today 12 people died at #Breitscheidplatz, 48 partially seriously injured are cared for by hospitals.

— PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016

"Our investigators assume that the truck was deliberately steered into the crowd at the Christmas market at Breitscheidplatz," police said on Twitter.

"All police measures related to the suspected terrorist attack at Breitscheidplatz are progressing at full steam and with the necessary diligence," police said.

The man who was found dead in the truck did not control the truck that drove to the Christmas market #Breitscheidplatz

— PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016

A man found dead in the truck was ruled out by police as the driver at the time of the attack. He died as a result of the crash and was later identified as a Polish citizen. The Polish delivery company that owned the vehicle said it lost touch with the driver at 4:00pm local time after it left the country for Berlin.

Witness video from #Berlin #Breitscheidplatz #truck pic.twitter.com/24tyJn3K3W

— Chris Koseloglou (@chriskose) December 19, 2016

The incident is reminiscent of the Nice truck attack in July when a man drove a 19-tonne truck along the beach front on Promenade des Anglais, mowing down people who had gathered to watch the fireworks on Bastille Day, killing 86 people. That attack was claimed by Islamic State.

Footage on social media appeared to show a number of ambulances at the scene, as well as depicting the damage left in the aftermath of the attack.

German Chancellor Angela Merkel has vowed to punish those responsible for the Berlin attack "as harshly as the law requires".

Her open-door policy on migration, which saw 890,000 asylum seekers arrive in Germany last year, has divided the country, with critics calling it a security threat.

Three separate terrorist attacks in July carried out by refugees have heightened tensions.