German police are hunting for the perpetrator of the truck attack at a Berlin market which killed 12, the BBC reprts.
Police released the only detained suspect on Tuesday, saying there was not enough evidence to hold him.
They now say more than one attacker may be on the run, possibly armed, and security has been stepped up in the German capital.
So-called Islamic State (IS) said one of its militants carried out the attack, but offered no evidence.
The former suspect, a Pakistani national named Naved B, had denied any connection with the attack. He was released by authorities who said there was no evidence he was involved.
Police said there were no forensic clues to link him to the vehicle, and had expressed doubt about his involvement soon after his arrest.