BERLIN (AFP) - A lorry ploughed into a busy Berlin Christmas market on Monday (Dec 19), killing 12 people in what German Chancellor Angela Merkel called a "terrorist attack".

A Pakistani asylum seeker was arrested shortly afterwards, but on Tuesday, police released him for lack of evidence, and the Islamic State in Iraq and Syria (ISIS) militant group claimed one of its "soldiers" carried out the killings.

Here is what we know of the attack, which was reminiscent of the horrific scenes in July when a truck drove into a crowd in the French Riviera city of Nice.

What happened

Around 8pm local time (3am Singapore time on Tuesday), a lorry drove for up to 80m into a busy Christmas market frequented by locals and tourists, smashing wooden stalls and crushing victims. Twelve people were killed and at least 48 injured.

A Pakistani asylum seeker held on suspicion of driving the Polish-registered truck was detained a short time later. But on Tuesday police released him after forensic tests failed to show he had been in the lorry's cab.

Police said a passenger was found shot dead in the cabin of the lorry and later identified him as a Polish national.

Terror attack

The deadly event took place in the shadow of the Kaiser Wilhelm Memorial Church whose damage in a World War II bombing raid has been preserved as a reminder of the horrors of war.

Chancellor Merkel said on Tuesday that the attack was a "terrorist act" and voiced fears - speaking before police backtracked on the Pakistani detainee - that the rampage was committed by an asylum seeker.

The interior ministry said there was no reason to close Germany's popular Christmas markets. "We must not let our free way of life be taken from us," it said in a statement.

The federal prosecutor's office has taken over the investigation, a routine step when terrorism is suspected.

A terrorist-linked news agency said on Tuesday that an ISIS "soldier" had carried out the attack "in response to appeals to target citizens of coalition countries".

The suspect

On Tuesday, Berlin's police chief warned that the "dangerous criminal" behind the attack is still at large.

"It is the case that we may have a dangerous criminal in the area and that of course makes the public nervous," said Mr Klaus Kandt, adding: "Of course we are boosting security measures... Heightened vigilance is also needed right now."

The Polish owner of the lorry confirmed on late on Monday that his driver was missing, while German officials said on Tuesday that a Polish man was found dead after the carnage was "shot" dead.

"One of the victims was shot," said Mr Karl-Heinz Schroeter, interior minister of neighbouring Brandenburg state, without specifying whether this victim was the registered driver of the truck.

The company's transport manager, Mr Lukasz Wasik, said the driver was 37 years old and had been transporting Thyssen steel products from Italy to Berlin.