ISIS says it inspired the attack at a Berlin Christmas market, but authorities say they still don't know who was behind the wheel when a truck plowed into a crowd, killing 12 people and injuring 48 others.
The ISIS-affiliated Amaq news agency released a statement Tuesday 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.
German authorities are investigating the attack as an act of terror. They released a man they'd earlier described as a suspect, saying there wasn't enough evidence to keep him in custody.
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," CNN terrorism analyst Paul Cruickshank said.
On Tuesday, German authorities released the man they'd detained after the attack, saying they haven't been able to prove that he was behind the wheel.
"The results of the investigation thus far have not produced imminent suspicion against the accused," Germany's 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," Peter Frank, general prosecutor at Germany's Federal Court of Justice, told journalists.
"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 and later released. German Interior Minister Thomas De Maiziere said earlier that the man was "probably from Pakistan." He entered Germany on December 31, 2015, 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.
A German intelligence official told CNN that security services were concerned that the attacker was still at large and dangerous.
Stalls shuttered at normally busy market
While investigators searched for clues, the usually bustling Christmas market was eerily quiet Tuesday, 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."
Thousands of people filled the nearby Kaiser Wilhelm Memorial Church to pay their respects to the victims Tuesday night.
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
Berlin police said the man found dead inside the truck was a Polish citizen and was not driving during the incident. The man appeared to have been shot dead, 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 at the time, according to Ariel Zurawski, the owner of the truck company.
Zurawski told CNN affiliate TVN 24 the vehicle may have been hijacked. He said his cousin -- the truck's regular driver -- couldn't have been behind the wheel.
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'
German Chancellor Angela Merkel visited the scene of the attack Tuesday afternoon, 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."
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.
'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.
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."
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."
De Maiziere said that Christmas markets would be closed for a day of mourning on Tuesday. "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.
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.
Both the US and UK governments had warned their citizens of potential security threats in Germany.
The US had issued a blanket travel warning for Europe, saying there was "credible information (which) indicates terrorist groups continue plotting possible attacks."