The Berlin Christmas market attack has been claimed by ISIL.
German prosecutors say a Pakistani asylum seeker who had been detained has been released.
The Chief Federal Prosecutor’s office said it did not have enough evidence. “The investigation up to now did not yield any urgent suspicion against the accused,” it said in a statement.
The 23-year-old had made extensive statements during a police hearing, but had denied the offence, it added.
A dozen people were killed and 48 injured, 18 of them seriously, when a lorry ploughed into stalls serving mulled wine and sausages in the heart of the German capital.
“It is possible that there is still a dangerous perpetrator on the run and of course people are worried. I believe people who live in this city should be vigilant,” Berlin’s police chief Klaus Kandt said during a news conference on Tuesday lunchtime.
He said police also thought it possible there might be more than one suspect.
The prosecutor’s office said in its later statement that it had been impossible to track the truck driver by eye-witnesses following the attack, and the investigation had not been able to prove that the suspect was in the truck’s cab at the time of the attack.
Police say a man found dead in the lorry was a Polish national, adding that he had not been in control of the vehicle. He is believed to be the original truck driver.
Germany’s interior minister Thomas de Maizière said a pistol believed to have been used to kill him had not yet been found.
Security measures are now being tightened around Christmas markets and other seasonal events in Berlin and elsewhere in Germany. Police with machine guns will patrol Christmas markets in the capital.
In Dresden concrete blocks were put in place to prevent vehicles from entering the market.
As Germany mourns, flags have been flying at half mast.
The authorities say people should not give in to terrorism by changing their plans – and should still attend seasonal events to celebrate Christmas.