Twelve people were killed, and dozens more injured, when a lorry careered through huts and stalls at a Christmas market near Kaiser Wilhelm Memorial Church, Berlin.
Here's what we learned of the attack on Tuesday, a day after the fatal incident.
The first victim of the attack has been identified.
Lukasz Urban, a 37-year-old Polish truck driver, appeared to have been stabbed and shot dead in his vehicle, which was thought to have been hijacked and driven into the crowds at the Christmas market.
Lukasz Wasik, the manager of the trucking company, described his colleague Urban as a "good, quiet and honest person" who was not only devoted to his work, but the vehicle he drove.
"I believe he would not give up the vehicle and would defend it to the end if attacked," Wasik said.
Urban, who was from the village of Roznowo, left behind a wife and teenage son.
The man police arrested, initially believing he could've been connected to the attack, has been released.
A Pakistani asylum seeker was arrested after the attack yet denied any involvement. It is understood he was been released due to a lack of evidence to link him to the attack and seek a formal arrest warrant to keep him in custody, German prosecutors have said.
The 23-year-old - who had arrived in Germany on December 31 last year - was released after a press conference in which federal public prosecutor Peter Frank said: "For now we don't know whether there was one attacker or several attackers. We also don't know whether they had support...
"We have to think that the person who was arrested yesterday, a man of Pakistani nationality, we have to be open to the idea that he could possibly not have been the attacker."
German police have said the person responsible for the attack may still be armed and at large.
A survivor of the attack spoke about the harrowing moment the truck careered into the crowds.
Inaki Ellakuria, a 21-year-old student from Spain, was one of the dozens injured at the Christmas market and immediately suspected it was no accident.
"I heard the truck crashing against the first stall," he tweeted on Monday.
"It came fast, too fast to be driving off the road accidentally," he posted.
His next tweet was chilling: "It has swept me and ran over both of my legs."
The economics student was passing through the area with two friends when the truck struck, and thanked the kind strangers who ran to help him.
"It hurts every time I think about it. I didn't think I was going to overcome the pain," he tweeted.
"Those who were not hurt were great with us. Cushions, water, food, blankets, hats, scarfs."
Ellakuria underwent surgery in Berlin for the broken tibia and fibula on his left leg. He also has broken bones in his right ankle and instep, as well as damage to his hip.
Islamic State has claimed responsibility for the lethal attack.
While German authorities treated the attack as one of terrorism, Islamic State claimed responsibility carried out by its "soldier".
In a statement from its Amaq news agency, the IS said that the attacker was "a soldier of the Islamic State and carried out the attack in response to calls for targeting citizens of the Crusader coalition".
Buckingham Palace is tightening its security.
While police revealed news plans would be put in place to protect people throughout the Christmas and new year period, extra security around Buckingham Palace will also be enforced for the Changing of The Guard ceremony.
Road closures will be in place for nearly two hours over the next three months for what police call "a precautionary measure", and are set to be in place by Wednesday.
Police said the change had always been planned but the process is being speeded up in the wake of the Berlin attack.