The German authorities say they cannot be sure if a man in custody was behind Monday`s lorry attack on a Berlin Christmas market that killed 12 people.
"We have to entertain the theory that the detainee might possibly not have been the perpetrator," federal prosecutor Peter Frank told reporters.
The style of attack and the target suggested Islamic extremism, he said.
The man detained, who has denied involvement, arrived in Germany from Pakistan at the end of last year.
He was captured in a park after reportedly fleeing the scene.
German Chancellor Angela Merkel, who laid flowers at the scene of the attack on Tuesday, vowed to punish those responsible "as harshly as the law requires".
Her open-door policy on migration, which saw 890,000 asylum seekers arrive in Germany last year, has divided the country, with critics calling it a security threat.
Several attacks carried out by refugees in July had already heightened tensions.
The lorry ploughed through the popular market at Breitscheidplatz, near west Berlin`s main shopping street, the Kurfuerstendamm.
Loaded with steel beams, it veered into the market at 20:14 local time (19:14 GMT), crashing through wooden huts and stands packed with tourists and locals.
Police believe the lorry drove 50-80 metres (160-260 ft) through the market area, Germany`s DPA news agency reports.
Interior Minister Thomas de Maiziere said 18 of the 48 people injured in the attack were in a serious condition.
What do we know about the suspect?
He arrived in Germany on 31 December of last year, turning up in Berlin in February, Mr de Maiziere said. His asylum application had not been completed.
Citing security sources, German media identified him as Naved B, 23, who was reportedly known to police for minor crimes, but not terrorist links.
Special forces stormed a hangar at Berlin`s defunct Tempelhof airport, where they believed he had been living in a shelter.
The suspect was seized near the Victory Column monument after allegedly fleeing the scene on foot for more than 2km (1.2 miles) towards the Tiergarten, a large public park.
Local newspapers say the driver of the lorry was followed by a bystander after jumping out of the cabin, and the man who tracked him used his mobile phone to keep in touch with police.
After Berlin police expressed doubt about whether they had in fact detained the right man, Mr Frank, the country`s public prosecutor general, said: "Currently we don`t know whether it was one or several perpetrators.
"We currently don`t know whether he or they had support."
Berlin police chief Klaus Kandt said it was possible that there was a "dangerous criminal on the loose" and he urged vigilance.
Where did the lorry come from?
Police say a Polish man, believed to be the original driver, was found shot dead on the passenger seat. No gun was found.
Ariel Zurawski, the Polish owner of the lorry, confirmed his driver had been missing since 16:00 (15:00 GMT) on Monday.
The lorry was registered in Poland but it is unclear whether it was travelling from Poland or returning from Italy, as some reports suggest.
What do witnesses say happened?
A British witness, Mike Fox, told the Associated Press news agency that the 25-tonne lorry had missed him by only about three metres as it smashed through stands.
"It was definitely deliberate," the tourist said.
Australian Trisha O`Neill told the Australian Broadcasting Corporation: "I just saw this huge black truck speeding through the markets crushing so many people and then all the lights went out and everything was destroyed."
`I heard the noise and the screams`
How has Germany reacted?
The interior ministry said Christmas markets in Berlin would remain closed on Tuesday but other markets outside of the capital would operate as normal.
Chancellor Merkel expressed her concern that the attacker might turn out to be an asylum seeker.
"I know that it would be particularly difficult for us all to bear if it turned out that the person who committed this act was someone who sought protection and asylum in Germany," she said.
The co-leader of Germany`s anti-immigration AfD party, Frauke Petry, blamed Mrs Merkel`s liberal policy on migrants.
"The milieu in which such acts can flourish has been negligently and systematically imported over the past year and a half," she said.
Horst Seehofer, the leader of Mrs Merkel`s sister party in Bavaria, urged the chancellor "to rethink our immigration and security policy and to change it".
Your questions on security in public spaces
Is this attack unprecedented?
Monday`s incident mirrored the lorry attack on Bastille Day crowds in the French city of Nice on 14 July, claimed by so-called Islamic State (IS).
A year of terror in Germany
Both IS and al-Qaeda have urged their followers to use vehicles as a means to attack crowds.
At the end of 2014, a van was driven into pedestrians at a Christmas market in the French city of Nantes, injuring 10 people.
Last week, German prosecutors said a 12-year-old German-Iraqi boy was in custody on suspicion of plotting to blow up a Christmas market in the western town of Ludwigshafen.