German police have searched a large shelter for asylum-seekers in Berlin in response to the fatal attack on Christmas market Monday.
The dpa news agency reported Tuesday that up to 250 officers took part in the operation at the now-defunct Tempelhof airport overnight.
It quoted a spokesman for Berlin's office for refugee affairs, Sascha Langenbach, saying four men in the late 20s were questioned by police but nobody was arrested.
Several German media, citing unnamed security sources, reported that the suspect in the attack was a Pakistani man who entered Germany late 2015 or early this year.
Police declined to comment on the reports, referring questions to federal prosecutors who didn't immediately respond to calls Tuesday.
Germany's top security official is ordering flags at federal buildings to be flown at half-staff in the wake of Monday's truck attack in Berlin.
Interior Minister Thomas de Maiziere said in a statement Tuesday that the decision was taken as a mark of sympathy following the attack.
Authorities say at least 12 people were killed and almost 50 injured when a heavily laden truck slammed into a Christmas market in the west of the German capital.