Police in Germany are working on the assumption that an armed attacker is still at large after doubts emerged over whether a man held in custody since Monday night was the suspected driver of a truck that rammed into a crowded Christmas market in Berlin, killing 12 people and wounding dozens more.

“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,” said Holger Münch, head of the Federal Criminal Police Office.

Berlin police said on Twitter they were being vigilant as the suspect had denied involvement in the incident.

Berlin Christmas market attack: prosecutors assume armed person still at large – live Read more

“The temporary arrested suspect denies the offence,” the force tweeted in English. “Therefore we are particularly alert. Please be also alert,” they urged the public.

Their comments were echoed by Peter Frank, the general state prosecutor, who told reporters: “We have to get used to the idea that the arrested man – a Pakistani – is very possibly not the perpetrator, and might not even belong to any circle of perpetrators.”

Frank added: “We don’t know if there was one perpetrator or several, or if there was any support.” But he underlined continuing belief about the nature of the attack, saying: “We are working on the basis this was an attack with a terrorist background.”

Berlin’s police chief, Klaus Kandt, announced that armed police in bullet-proof vests would be posted at the entrances to all Christmas markets across Germany and that extra stone barriers would be erected.

Doubts over the detained man were first reported in the German newspaper Die Welt, which quoted a high-ranking security source who said: “We have the wrong man, and therefore the situation has changed. It means the real perpetrator is still armed and still at liberty, and can cause damage.”

All Berlin patrol police and special forces have been informed, the newspaper reported, quoting the security source.

The suspect taken into custody shortly after the attack was a 23-year-old of Pakistani origin who arrived in Germany last year. He has denied any involvement, the interior minister, Thomas de Maizière, said.

At a press conference on Tuesday morning, the German chancellor, Angela Merkel, said: “We have to assume we are dealing with a terrorist attack.”

“It would be particularly hard to bear for all of us if it was confirmed that a person committed this crime who asked for protection and asylum in Germany,” she added. “This would be particularly repugnant in the face of the many, many Germans who have dedicated themselves day after day to helping refugees, and in the face of the many people who actually need our protection and try to integrate into our country.”

The death toll rose overnight from nine to 12, as doctors in clinics around Berlin worked to save lives and treat injuries, including many cases of internal bleeding in people who were crushed by the truck.

The Polish-registered truck careered into the busy Breitscheidplatz market at about 8pm (1900 GMT) on Monday. It had a run-up of about 80 metres before crashing into stalls and shoppers outside the Kaiser Wilhelm memorial church.

A witness told the Guardian the lorry hit the market at speed. “It was not an accident. The truck was doing 40mph. It was in the middle of a square, there are main roads either side [where it could have come from]. But it showed no sign of slowing down,” said Emma Rushton, a British tourist.



She said the truck crashed into a stall only a few feet from where she and her friend were standing. “We heard a massive bang,” said Rushton. “About 8-10ft in front of us was where the lorry ploughed through. It ploughed through the stall where we bought our mulled wine.



Facebook Twitter Pinterest Angela Merkel visits the Kaiser Wilhelm memorial church on Tuesday. Photograph: Hannibal Hanschke/AFP/Getty Images

“It ploughed through people and the wooden huts, it tore the lights down. Everything went dark, it was black and there was screaming. It was awful.”

One of the dead was the vehicle’s registered driver, a Polish national who police said was shot in Potsdam, about nine miles west of Berlin, before the market attack. The Polish delivery company that owns the Scania lorry said it lost touch with its driver at 4pm local time after it crossed from Poland into Germany.

Merkel and other senior politicians visited the attack site on Tuesday afternoon, laying white flowers in front of the church.