German police say they are unsure if a man they have in custody was behind Monday’s lorry attack in Berlin that killed 12 and wounded 48 others.
“It is in fact uncertain whether that really was the driver,” Berlin police head Klaus Kandt said on Tuesday.
The man detained, who has denied involvement, arrived in Germany from Pakistan at the end of last year.
He was captured in a park 2km away after reportedly fleeing the popular Christmas market in west Berlin.
Mrs Merkel has vowed to punish those responsible for the Berlin attack “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.
Three separate terrorist attacks in July carried out by refugees have heightened tensions.
The lorry ploughed through the popular market at Breitscheidplatz, near west Berlin’s main shopping street on Monday evening.
The truck, which was loaded with steel beams, veered into the market at 20:14 local time (19:14 GMT), one of its busiest times. It crashed through wooden huts and stands packed with tourists and locals.
The DPA news agency said police believe the lorry drove 50-80 metres (160-260 ft) through the market area.
Interior Minister Thomas de Maiziere said 18 of the 48 people injured were in a serious condition.
What do we know about the suspect?
Doubts are being cast over whether the suspect in custody was the driver of the lorry, with Berlin police tweeting for people to stay alert.
Mr de Maiziere said the arrested man had arrived in Germany on 31 December last year, and later turned up in Berlin in February. His asylum application had not been completed, he added.
German media have identified the man, citing security sources, as a 23-year-old named Naved B.
He was reportedly known to police for minor crimes, but not terror links.
Special forces earlier stormed a hangar at Berlin’s defunct Tempelhof airport, where they believed he had been living in a shelter.
He was seized after leaving the lorry and fleeing on foot for more than a mile (2km) towards the Tiergarten, a large public park.
A witness who followed him called the police, who quickly detained the suspect near the Victory Column monument.
Where did the lorry come from?
Police said a Polish man, believed to be the original driver, had been found dead on the passenger seat.
Ariel Zurawski, the Polish owner of the lorry, confirmed that his driver was missing and had been unreachable since 16:00 (15:00 GMT) on Monday.
The truck was registered in Poland, but it is unclear whether it was travelling from Poland or returning from Italy, as some reports suggest.
How has Germany reacted?
Chancellor Merkel said she was “shocked and very saddened” by the attack but added “we don’t want to live with fear of evil”.
Germany’s interior ministry said Christmas markets in Berlin would remain closed on Tuesday but other markets outside of the capital would operate as normal.
A senior member of Germany’s anti-immigration AfD party, Marcus Pretzell, blamed Mrs Merkel for the attack, linking it to her open-door migration policy.
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” in the wake of the attack.
What do eyewitnesses say happened?
A British eyewitness, Mike Fox, told Associated Press that the 25-tonne lorry had missed him by only about three metres as it smashed through stands.
“It was definitely deliberate,” said the tourist.
He said he had helped people who appeared to have broken limbs, and that others were trapped under Christmas stands.
Australian Trisha O’Neill told the Australian Broadcasting Corporation she witnessed “blood and bodies everywhere”.
“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.”
Is this the first such attack?
A series of small-scale attacks by Islamist militants alarmed Germany this year. Ten people were killed and dozens more injured in separate gun, bomb, axe and machete attacks in Bavaria and Baden-Wuerttemberg in July.
But Monday’s incident was reminiscent of the lorry attack on Bastille Day crowds in the French city of Nice on 14 July, claimed by so-called Islamic State (IS).
The mayor of Nice, Philippe Pradal, said the Berlin incident shared the same “blind violence” as the attack on his city.
Both IS and al-Qaeda have urged their followers to use trucks as a means to attack crowds.
US President-elect Donald Trump blamed “Islamist terrorists” for a “slaughter” of Christians in the German capital.
“Today there were terror attacks in Turkey, Switzerland and Germany – and it is only getting worse. The civilized world must change thinking!” he tweeted.
President Francois Hollande meanwhile said there was “a high level of threat” in France from a terror attack after Berlin.