The Brandenburg Gate in Berlin is illuminated in the colors of the German flag on Tuesday, December 20, one day after a truck crashed into a crowded Christmas market there. At least 12 people were killed and 48 injured in what police are investigating as a terrorist attack. Sean Gallup/Getty Images

A police officer places a candle at a makeshift memorial in Berlin on December 20. Sean Gallup/Getty Images

A woman is illuminated by candles as she cries in Berlin on December 20. Markus Schreiber/AP

A man prays at a Berlin memorial on December 20. JOHN MACDOUGALL/AFP/Getty Images

German Chancellor Angela Merkel lays flowers at the memorial on December 20. She is joined by, from left, Berlin Mayor Michael Muller, Minister of the Interior Thomas de Maiziere and Foreign Minister Frank-Walter Steinmeier. Michael Kappeler/ZUMAPRESS.com

Merkel, bottom left, views the scene of the attack. Markus Schreiber/AP

A woman reacts near the crime scene on December 20. Matthias Schrader/AP

Mourners gather at the Kaiser Wilhelm Memorial Church near the crash site. MAURIZIO GAMBARINI/DPA/EPA

A police officer holds her weapon near the market. TOBIAS SCHWARZ/AFP/Getty Images

A woman lays flowers at a makeshift memorial across from the market. Sean Gallup/Getty Images

A stall worker is comforted in the market area on December 20. Sean Gallup/Getty Images

People attend a memorial service at St. Hedwig Cathedral in Berlin on December 20. Michael Sohn/AP