Last night, a lorry ploughed into shoppers at a busy Christmas market outside Kaiser Wilhelm Memorial Church in the heart of Berlin.
This is what we know so far.
Twelve people have died.
The number rose from nine given by officials last night. A further 48 people were injured, some seriously.
German officials believe a passenger in the lorry, a Polish man, was shot dead.
Witnesses described the scene as 'carnage'
US tourists Luke Theis, 21, and his girlfriend Lara Colombo, 22, were on the other side of Kaiser Wilhelm Memorial Church and were heading to the area which was targeted.
Theis said: "We started seeing people running and hearing ambulances from all directions so we walked over. It was carnage everywhere. There was blood all over the floor.
"There were people lying on the floor - I am not sure what their condition was - I could count about eight lying down.
"The biggest mental image I have is there were two rivers of blood going down the floor."
It's being treated as a suspected act of terrorism.
Police in Berlin have said the driving of the truck into the crowds was intentional.
The driver is believed to be an asylum seeker.
Authorities said they arrested a "suspicious person" near the scene, who was thought to be the driver.
Security sources believe he is a 23-year-old Pakistani asylum seeker named Navid B who lived in a hangar at Berlin's old Tempelhof airport and reportedly arrived in Germany earlier this year on February 11 after crossing the border in Passau, Bavaria.
The man who was arrested has denied involvement, according to Germany's top security official.
Angela Merkel said it would be "very difficult" if the attacker is a refugee.
The German Chancellor has said she was "shocked and very saddened" by the attack and that "it would be very difficult for us to learn that a human being committed this deed who came to Germany to ask for refuge and asylum".
But she added: "We will find the strength to continue living life as we want to live it in Germany - in freedom, openness and together."
UK police are now reviewing their own security.
The Metropolitan Police said they have plans in place to keep people attending events over Christmas and New Year safe.
The force said: "These already recognise that the threat level is at 'severe', meaning an attack is highly likely, and have considered a range of threats, including the use of large vehicles.
"As a matter of routine, as a precaution, we review our plans after attacks overseas, and we are doing so at present following the awful incidents in Berlin and Ankara last night."