DAESH has claimed responsibiliy for the terror attack on a Berlin Christmas market that killed 12 people.

The extremist group said through its new agency that one of its “soldiers” was responsible.

Earlier, German prosecutors released a man who was initially arrested – saying there was insufficient evidence to tie him to the rampage – and admitted the attacker might still be on the run. The original suspect, a Pakistani citizen who moved to Germany last year as an asylum seeker, denied involvement in the attack that also injured nearly 50 others.

They said witnesses were able to follow the truck driver from the scene but lost track of him. The man arrested matched witness descriptions of the driver, but investigators could not prove he was in the lorry’s cab when it ploughed into shoppers and revellers.

Peter Frank, the country’s most senior prosecutor, said it was not clear whether one person or more were responsible for Monday night’s carnage. He acknowledged the method used was reminiscent of the attack in Nice in July when a 19-tonne lorry was driven into tourists walking along the Promenade Anglais, killing 86 people, including 10 children.

The head of Germany’s federal criminal police office, Holger Muench, said the authorities remained on high alert and were investigating “in all directions”.

Muench said six of the people who died have been identified and were German, but details of the other six victims have not yet been confirmed.

The first victim in the attack was a 37-year-old Polish lorry driver who was seemingly stabbed and shot to death in the cabin of his truck, which was hijacked by the attacker or attackers.

One of Lukasz Urban’s colleagues said he was so dedicated to his work and his truck that he would have defended his vehicle “to the end”.

As security increased at Christmas markets yesterday, eyewitnesses told of being caught up in the horror.

Newspaper editor Jan Hollitzer described hearing cries and seeing people hurt among the destroyed stalls, the broken glass and smashed crockery. “I heard the noise and the screams,” he said. “I saw some people lying on the ground and also some people under the truck. It was really horrible and as chaotic as you can imagine.”

Berliner Ibrahim Colak was with a group of friends and about to leave the market when the attack happened. “We heard a sound as if one of the Christmas market stalls was falling down,” he said. “When I turned, I saw the lorry was going full speed. I believe it was definitely an attack. It wasn’t stopping or trying to manoeuvre out of the way.”

Colak said he removed wood that had fallen on some of the people in the market.

He added: “I was trying to help at the back of the lorry the people as well, but it was all, oh God, it was just horrible to describe.”

Tourist Rhys Meredith from Cardiff was at the market with his girlfriend. “We were having a good walk around, soaking up the scene, soaking up the atmosphere,” he said.

“We had bought food and had we carried on we would have walked down the path the truck would have been down and we more than likely wouldn’t be having this conversation right now.”

Meredith said the lorry caused “complete and utter devastation”.

He said: “Myself, along with a group of locals, had tried pushing collapsed stalls off of the top of people who had already died. Unbelievable scenes, I can’t really put into context. My girlfriend looked after a couple of people with very serious head injuries.”

Chancellor Angela Merkel and other senior German officials laid white roses at the sombre scene.

In a statement on German television, Merkel described herself as “shocked, shaken and deeply saddened” by what had happened.

She said: “We must, as things stand, assume it was a terrorist attack.”

Merkel has been criticised by figures on the right both within and outside her party for allowing large numbers of refugees into Germany and, speaking before the suspect’s release, she addressed the possibility an asylum seeker was responsible.

“I know that it would be particularly hard for us all to bear if it were confirmed that a person committed this act who asked for protection and asylum in Germany,” she said. “Even if it’s difficult, we will find the strength to live life the way we want to live it in Germany. Free, together and openly.”

Far-right groups seized on the incident, blaming Merkel for what had happened. “Under the cloak of helping people Merkel has completely surrendered our domestic security,” Frauke Petry, the co-chairwoman of the Alternative for Germany party, wrote.