BERLIN — Germany must assume a truck plowing through a crowded Christmas market in Berlin was a “terrorist attack,” Chancellor Angela Merkel said Tuesday, while authorities expressed uncertainty over whether they had arrested the correct suspect.

Twelve people were killed and nearly 50 others injured when the truck drove into the popular Christmas market filled with tourists and locals outside the Kaiser Wilhelm Memorial Church near Berlin’s Zoo station late Monday.

Police detained an asylum-seeker from Pakistan shortly afterward, but he denied involvement, Interior Minister Thomas de Maiziere said. The man had entered Germany on Dec. 31 last year and arrived in Berlin in February.

Berlin police chief Klaus Kandt said Tuesday it wasn’t clear whether the man detained was really the driver.

“We haven’t been able to confirm it yet,” he told reporters.

Berlin police urged people to remain “particularly vigilant” and to report “suspicious movement” to a special hotline.

In a statement widely broadcast on German television, Merkel described herself as “shocked, shaken and deeply saddened.”

“There is still a lot that we don’t know about this act with sufficient certainty,” she said. “But we must, as things stand, assume it was a terrorist attack.”

Merkel, who has been criticized for allowing in large numbers of migrants, addressed head-on the possibility that an asylum-seeker was responsible.

“I know that it would be particularly hard for us all to bear if it were confirmed that a person committed this act who asked for protection and asylum in Germany,” Merkel said. “This would be particularly sickening for the many, many Germans who work to help refugees every day and for the many people who really need our help and are making an effort to integrate in our country.”

Authorities arrested a man about 2 kilometers (1½ miles) from the crash site on suspicion of having been at the wheel of the truck. Footage showed the suspect, his head covered in a white sheet, being pushed into a police car shortly after the attack. Berlin’s Tagesspiegel newspaper reported that the man was known to police for minor crimes.

A spokesman for Berlin’s office for refugee affairs said police conducted a large-scale search overnight at a large shelter for asylum-seekers at the city’s now-defunct Tempelhof airport. Four men in their late 20s were questioned but nobody was arrested, Sascha Langenbach told The Associated Press.