Ms. Merkel, dressed in black, made a brief appearance before reporters Tuesday morning, saying that she was “horrified, shaken and deeply sad.” Those behind the killings would be punished “as severely as our laws demand,” Ms. Merkel said.
In the attack, a tractor-trailer truck jumped a sidewalk around 8 p.m. and plowed into the market near the Kaiser Wilhelm Memorial Church, a symbolic Berlin site whose spire, jagged from bomb damage, was intentionally left unrepaired after World War II.
The driver fled after the attack.
The police said they had later arrested a man near the scene who was suspected of involvement. The chief of police in Berlin, Klaus Kandt, told reporters on Tuesday that “it is actually not clear” whether the man they had arrested was the driver.
The chief federal prosecutor, Peter Frank, noted at a news conference that “a suspect is not a perpetrator,” and said the investigation of the man was a priority, but not the only one. He refused to speculate about whether there were others involved in the attack who were still at large.
Mr. de Maizière said in a late-morning news conference that the suspect entered Germany and registered as an applicant for asylum on Dec. 31, 2015, and that he reached Berlin in February. Several hearings were scheduled in his asylum case, Mr. de Maizière said, but the man did not appear at some hearings and there were problems with translation at others, so his application has not been processed.
Mr. de Maizière said the suspect had denied any involvement in the attack.
Officials in Berlin have been straining to deal with a flood of asylum applications. Although the number of arrivals has slowed recently from a high point in the summer of 2015, tens of thousands remain in communal housing, awaiting processing of their applications.
Merkel: ‘This Is a Very Difficult Day’, Chancellor Angela Merkel of Germany said that, for now, it must be assumed that a truck's plowing into a Christmas market in Berlin on Monday night, killing 12 people, was an act of terrorism.
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In addition to the 12 dead, 48 people were wounded at the Berlin market, 18 of them critically, Mr. de Maizière said.
Seeking clues about the attacker and his motives, the German special police conducted a search at the refugee shelter at Tempelhof Airport in Berlin early Tuesday, a security official said, declining to give details.
The Berlin police appealed to anyone who was at the Christmas market or in the area on Monday night to send them any videos or photographs of the attack and to refrain from posting the material on social media.
Mr. de Maizière made a similar appeal, noting that the country’s security depended on tips from the public.
The president of the federal criminal police, Holger Münch, said investigators were collecting evidence at the scene and were asking the public for any information that could help piece together the sequence of events leading to the attack.
Mr. Münch noted that his agency had warned of a possible terrorist attack, and that the events in Berlin had confirmed that the threat was serious.
The German newsmagazine Der Spiegel said the search of the shelter at Tempelhof occurred around 4 a.m. No arrests were made, according to the radio station Rundfunk Berlin-Brandenburg, which cited a city security official whom it did not identify.
“Twelve people were among us yesterday and were happy about Christmas and the holidays,” Ms. Merkel said. “This is incomprehensible, this act that robbed them of their lives.”
She said she was meeting with Germany’s interior and justice ministers and would visit the Christmas market later in the day. “We don’t want to do without Christmas markets, without nice outings together,” she said. “We do not wish to let fear and angst take away our freedom to live.”
Mr. de Maizière said that Berlin’s Christmas markets should close for a day, but he added that, in general, Germany’s many holiday-themed markets and other events should continue to take place. He said that there would be an increased security presence and that local officials would make decisions about particular aspects of events that might need to be curtailed for safety reasons. He also said that security would be heightened at railway stations and other public facilities.
Officials have been aware for some time of the risk of attacks on holiday-themed events in Europe. The State Department issued a travel alert on Nov. 21 recommending that Americans “exercise caution at holiday festivals, events, and outdoor markets” throughout the Continent.
Witness Describes Fatal Crash At Berlin Market; a truck plowed into a Christmas market in Berlin on Monday evening, leaving at least twelve people dead and many more injured, according to the police. A British tourist who witnessed the incident described what she saw.
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“Credible information” indicated that the Islamic State, its affiliates and sympathizers “continue to plan terrorist attacks in Europe, with a focus on the upcoming holiday season and associated events,” the alert said, and that an attack could come with little or no warning.
The general mood in Berlin was subdued on Tuesday. People went about their business calmly but much more quietly than usual, and they were reluctant to speak to reporters — or, seemingly, even to one another.
“People are a little bit taking time off, and a little bit afraid,” said Memo El-Schafie, 47, a vendor at a coffee and cake stand in the Stadtmitte subway station, where there were notably few passengers for a weekday morning.
Heiko Maas, the German federal justice minister, said on Tuesday that the events “not only hit Berlin right in its heart, but hit us all.”
The truck that plowed into the market belonged to a Polish company, and more signs emerged on Tuesday that it had been hijacked.
Mr. de Maizière said that one of the 12 people killed in Berlin was a Polish man who was found dead with a gunshot wound in the passenger seat of the truck. The gun that caused the injury has not been recovered, he said. Officials indicated that they considered the Polish man a victim and not a perpetrator.
Frauke Petry of the Alternative for Germany party said in a statement early Tuesday that “Germany is no longer safe,” and told citizens that it would be Ms. Merkel’s “duty to tell you that.”
Noting the successive terrorist attacks in France, including a truck driven into a crowded beachfront promenade in Nice in July, Ms. Petry called the carnage at the Berlin market “not just an attack on our freedom and our way of life but also on our Christian tradition.”
The side of the market where the truck slammed into the crowd remained cordoned off early Tuesday. Police officers patrolled the area, as Berlin residents bearing flowers and candles placed them at makeshift memorials on either side of the church.