The German interior minister, Thomas de Maizière, said that a man arrested on suspicion of involvement in the attack was a 23-year-old Pakistani who had applied for asylum.
“We must assume at the current time that it was a terrorist attack,” Ms. Merkel said on Tuesday. “I know that it would be particularly difficult for all of us to bear if it is confirmed that this deed was carried out by a person who sought protection and asylum in Germany.”
It would also greatly amplify the political problems Ms. Merkel already faces over her government’s policy of admitting refugees by the hundreds of thousands. The policy has come under mounting criticism, both from her allies and from the far right of the political spectrum.
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 was driven by the perpetuator over a sidewalk around 8 p.m., who plowed it 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.
Witnesses saw one person, presumably the driver, exit the truck, and some of them followed the fleeing person and notified the police, prosecutors said.
The police 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 he said the investigation of the man was a priority, but not the only one in the case. He added that the authorities did not yet know whether any group was behind the attack or if anyone else was involved.
According to Mr. de Maizière, the suspect entered Germany and registered as an applicant for asylum on Dec. 31, 2015, and reached Berlin in February. Several hearings were scheduled in his asylum case, Mr. de Maizière said, but the man did not appear at some of them, and there were problems with translation at others. As a result, 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.
Frauke Petry of Alternative for Germany, a far-right opposition party that has been gaining strength, said in a statement early Tuesday that “Germany is no longer safe.”
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.”
(Source: NYT, Daily Sun)