BERLIN -- Police say the man they initially detained after a truck plowed into a Berlin Christmas market may not have been the driver, leading to fears that the attacker could still be on the loose.
Now, the man they detained has been released due to insufficient evidence against him.
"The investigations thus far have not produced urgent suspicion against the suspect," Germany's general prosecutor said in a statement Tuesday.
"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.
German authorities had earlier described an asylum-seeker as a suspect in the Monday evening attack, which they're investigating as an act of terror. But now, a key question in the investigation apparently remains unanswered: Who was behind the attack that left 12 people dead and 48 others injured?
So far, no group has claimed responsibility.
Officials say several people could have been involved in the attack. A weapon used in the attack has not yet been found. The truck is owned by a Polish company. A Polish man was found dead in truck's passenger seat. Chancellor Angela Merkel visited the site where 12 people died.
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."
Attacker at large?
Berlin Police President Klaus Kandt said Tuesday that officials could not be certain that the man in custody -- who was picked up about a mile away from the market -- was responsible for the attack.
German Interior Minister Thomas De Maiziere said the man, who he said was "probably from Pakistan," had denied any involvement in the attack.
The man had entered Germany on December 31, 2015, De Maiziere said, and his application for asylum had stalled.
Berlin police have called on the public to remain alert as confusion swirled over who might have been behind the wheel when the truck barreled over a curb into the crowded market.
"We possibly need to assume that we have not arrested the right one," General Prosecutor at Germany's Federal Court of Justice Peter Frank told journalists Tuesday.
"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."
Asked at the same news conference whether the driver might still be at large, President of the Federal Criminal Office Holger Münch said only that authorities had one man in custody and that they were "looking in all directions" for any other suspects.
He also said a weapon believed to have been used in the attack had not yet been found.
A German intelligence official told CNN that security services were concerned that the attacker was still at large and dangerous.
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 added 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 Maizier 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 bears resemblance 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."