Flowers and candles are placed beside a placard reading "The heart of Berlin is hit" at the attacked Christmas market to mourn the victims in Berlin, capital of Germany, on Dec. 21, 2016. German police is searching nationwide for a 23-year-old man under suspect after a lorry ploughed into a crowd at a Christmas market in Berlin on Monday night, killing 12 people and injuring many others. (Xinhua/Shan Yuqi)