'Back to square one': Berlin is on high alert as authorities search for a culprit still at large
One suspect, a Pakistani asylum-seeker who was known to German police, was released Tuesday due to a lack of evidence, and residents have been urged to remain on guard.
"It is the case that we possibly still have a dangerous offender in our area," Berlin police chief Klaus Kandt told media on Tuesday. "These days it is necessary to be vigilant."
ISIS claimed credit Tuesday afternoon for the attack, confirming what German and US officials had presumed was an act of terrorism.
The group's Amaq news agency declared the perpetrator to be an ISIS soldier who "executed the operation in response to calls to target nationals of the coalition countries."
It's unclear whether the perpetrator or perpetrators acted on their own, or coordinated with ISIS leadership.
Investigators remain "highly alarmed" at the lack of leads in the case, and are wading through witness descriptions and surveillance footage and ramping up efforts to process forensic evidence such as the bloodstains found inside the truck, according to The Washington Post.
But police have not yet released any sketches of potential suspects to the public, nor have they recovered the weapon used to kill the Polish man whose body was found in the truck's passenger seat.
"They're really back to square one in terms of this investigation," CNN terrorism analyst Paul Cruickshank said.
"It may well be a scenario of a manhunt, a race against time to arrest this individual before they can strike again."
The confusion surrounding the lack of a suspect is "completely unprecedented," one terrorism expert told CNN.
"You have peace concerts, you have people laying down wreaths, you have people writing cards and people speaking silent prayers," said Peter Neumann, director of the International Center for the Study of Radicalization. "I don't think a lot of people are aware there's an attacker on the loose, potentially."