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Berlin (CNN) German authorities released a man and intensified their manhunt for the person who plowed a tractor-trailer truck into a crowd at a Berlin Christmas market , even as ISIS claimed Tuesday that it inspired the attack.

Police said they didn't have enough evidence to hold a man they'd earlier described as a suspect in the Monday night truck assault that left 12 people dead and 48 injured. The dead include a man found shot inside the truck.

There may be more than one suspect at large, armed and dangerous, Peter Frank, general prosecutor at Germany's Federal Court of Justice, told journalists.

The ISIS-affiliated Amaq news agency released a statement saying Monday night's attack was carried out by "a soldier of the Islamic State" in response to calls by the group's leadership to target citizens of international coalition countries.

CNN terrorism analyst Paul Cruickshank said ISIS often uses this terminology to refer to attacks by alleged sympathizers in the West.

"This should not be taken to mean the group is claiming it directed this attack," Cruickshank said. "Investigators have not uncovered any links to ISIS."

German authorities are investigating the attack as an act of terror. Chancellor Angela Merkel told journalists that we must "assume this is a terrorist attack."

Latest developments

Prosecutor: Forensic tests haven't confirmed man was in truck's cabin.

Mourners pack nearby church at memorial for victims.

Officials say several people could have been involved in the attack.

A weapon used in the attack has not yet been found.

Chancellor Angela Merkel visited the site where 12 people died.

Attacker at large?

Berlin police called on the public to remain alert as confusion swirled over who was driving when the truck barreled over a curb into the crowded market.

"They're really back to square one in terms of this investigation. ... It may well be a scenario of a manhunt, a race against time to arrest this individual before they can strike again," Cruickshank said.

German authorities released the man they'd detained after the attack. "The results of the investigation thus far have not produced imminent suspicion against the accused," the general prosecutor said in a statement.

Investigators had initially described the asylum seeker they detained as a suspect. But a day later, they seemed to be backing off that claim.

"We possibly need to assume that we have not arrested the right one," Frank said.

"We do not know if there was one perpetrator or several perpetrators yet. We do not know if there was support given to the perpetrator."

Authorities didn't release the identity of the man they detained. De Maiziere said earlier that the man was "probably from Pakistan." He entered Germany on December 31, 2015, Interior Minister Thomas De Maiziere said, and his application for asylum had stalled.

So far, forensic tests haven't linked the man to the truck's cabin, the prosecutor said.

Berlin police turned to the public for help and tweeted that they'd received 508 leads. Police also asked citizens to upload any digital video and photos of the incident or possible suspects.

Bitte übersenden Sie Ihre Bild- und Videohinweise zum Ereignis #Breitscheidplatz an folgendes Upload-Portal:https://t.co/VgRGAV8AJB#Danke — PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016

Stalls shuttered at normally busy market

While investigators searched for clues, the usually bustling Christmas market was eerily quiet, with stalls shuttered and nearby roads blocked off.

The sounds of a choir singing rang out as a terrorism expert spoke to CNN near the market.

"You have peace concerts, you have people laying down wreaths, you have people writing cards and people speaking silent prayers. This is completely unprecedented," said Peter Neumann, director of the International Center for the Study of Radicalization. "I don't think a lot of people are aware there's an attacker on the loose, potentially."

Those unable to enter the packed building waited outside in the winter air, many hugging each other, crying, or silently clutching candles.

Polish citizen found dead

Police identified the man found shot to death inside the truck as the original driver from Poland, reported German state media ARD.

He was not driving when the truck came through the market, police said. The gun has not been recovered, De Maiziere said.

The truck, which was owned by a Polish company, "was steered deliberately into the crowd," police said. It was carrying 25 tons of steel, according to Ariel Zurawski, the owner of the truck company. Zurawski told CNN affiliate TVN 24 the vehicle may have been hijacked.

Zurawski identified the slain man as his cousin, ARD said.

Photos: Truck plows through Berlin Christmas market Authorities examine a truck Tuesday, December 20, that crashed into a crowded Christmas market in Berlin the night before. At least 12 people were killed and 48 injured in what police are investigating as a likely terrorist attack. Hide Caption 1 of 18 Photos: Truck plows through Berlin Christmas market Emergency responders tow the truck's cab away from the crash site on December 20. Ariel Zurawski, owner of the truck company, said the truck may have been hijacked as it was transporting 25 tons of steel. Hide Caption 2 of 18 Photos: Truck plows through Berlin Christmas market An overview of the crash site on December 20 shows where the tractor-trailer drove over the sidewalk and into market stalls near the Kaiser Wilhelm Memorial Church. Hide Caption 3 of 18 Photos: Truck plows through Berlin Christmas market A rescue worker stands beside Christmas decorations that were scattered by the crash. Hide Caption 4 of 18 Photos: Truck plows through Berlin Christmas market Members of the press crowd around the crash site on December 20. Hide Caption 5 of 18 Photos: Truck plows through Berlin Christmas market A rescue worker looks at destroyed market stalls near the truck's trailer. Hide Caption 6 of 18 Photos: Truck plows through Berlin Christmas market Authorities investigate the crash site on December 20. Hide Caption 7 of 18 Photos: Truck plows through Berlin Christmas market Emergency responders treat an injured person on Monday, December 19. Hide Caption 8 of 18 Photos: Truck plows through Berlin Christmas market German police shine a flashlight onto the back of the truck. Hide Caption 9 of 18 Photos: Truck plows through Berlin Christmas market Emergency crews work near the crash site on December 19. Hide Caption 10 of 18 Photos: Truck plows through Berlin Christmas market Police officers stand next to the truck. Hide Caption 11 of 18 Photos: Truck plows through Berlin Christmas market Ambulances are seen where the incident happened in western Berlin. Hide Caption 12 of 18 Photos: Truck plows through Berlin Christmas market Police stand beside the damaged truck that crashed into the market. Hide Caption 13 of 18 Photos: Truck plows through Berlin Christmas market First responders load an injured person into an ambulance. Hide Caption 14 of 18 Photos: Truck plows through Berlin Christmas market A Christmas tree lies next to the truck. Hide Caption 15 of 18 Photos: Truck plows through Berlin Christmas market Police take someone into custody near the market. Hide Caption 16 of 18 Photos: Truck plows through Berlin Christmas market Police stand guard at the market after the crash. Hide Caption 17 of 18 Photos: Truck plows through Berlin Christmas market An armed police officer stands near the scene. Hide Caption 18 of 18

Scene of the Christmas market attack in Berlin.

Zurawski said that his cousin was happy to be finishing his route from Italy to Berlin and was looking forward to returning to Poland after one last leg to Denmark.

"He was asking if he should be back home by Thursday night because he still needed to buy a Christmas gift for his wife," he said.

Merkel: 'Hard to bear'

saying earlier it would be "especially disgusting" if anyone in the attack had been given asylum. Merkel visited the scene of the attack,saying earlier it would be "especially disgusting" if anyone in the attack had been given asylum.

Dressed in black and making her first public comments about the incident late Tuesday morning, she called for unity in the country.

"I know that it would be especially hard to bear for us if it was to be confirmed that a person (who) committed this act ... was given protection and asylum in Germany," Merkel said.

"This is a very difficult day. I, like millions of people in Germany, am horrified and deeply sad about what happened yesterday in Berlin."

German Chancellor Angela Merkel arrives to give a statement the day after the Berlin attack.

Monday's attack could cause further political upheaval for Merkel, who has come under criticism over her government's generous acceptance of refugees. Germany has taken in more than 890,000 asylum-seekers in the past year, a much higher number than other European nations.

But a backlash has been growing, fueled in part by Islamist terror attacks in Germany and across the continent.

World leaders, including US President Obama and French President Francois Hollande, expressed support for Merkel and the German people.

'It felt like slow motion'

Before the attack on Monday evening it was a quintessential German Christmas scene at Breitscheidplatz: Trees strung with lights, vendors serving candied fruit and waffles, the smell of gluhwein -- German mulled wine -- wafting through the cold December air.

Authorites inspect a truck that sped into a Christmas market in Berlin.

American Shandana Durrani was at the market, at the foot of the Keiser Wilhelm Memorial Church, and had stopped to reply to a text message when the truck rammed into the crowd at around 8 pm.

She was lucky to have stopped, she said, as the truck mounted the curb, mowing people and stalls down just 20 feet in front of her, sending everyone "running, scurrying, screaming."

"I heard some popping and thought maybe there was a guy with a gun," she told CNN.

"People just started running and dropping their gluhwein."

View of the truck that crashed into a Christmas market at Gedächtniskirche church in Berlin.

She said that it looked as if the driver had just mounted the curb and lost control, and that the whole thing probably lasted a mere 10 seconds.

"It probably didn't last very long, but it felt like it was in slow motion (as I ) tried to get away from it."

Security concerns

De Maiziere said that Christmas markets would be closed Tuesday for a day of mourning. "But to cancel them would be wrong," he said, adding they would hopefully reopen the following day.

Münch, of the Federal Criminal Office, said that authorities had assessed the likelihood of an attack at Christmas markets recently.

Security officers guard the area after the Berlin Christmas market attack.

The attack is similar to one in Nice, France, in July, when a truck rammed into a crowd gathered to see Bastille Day fireworks, killing 86 and injuring more than 200 people

Terror groups including ISIS and a branch of al Qaeda have encouraged their followers to use vehicles to stage attacks

The US and UK governments had warned their citizens of potential security threats in Germany.