A man arrested on suspicion of killing 12 people by driving through a Berlin Christmas market in a truck has been released because of a lack of evidence, according to the chief federal prosecutor's office.
The man released was a young Pakistani refugee who had arrived in Germany in February.
"The investigation up to now did not yield any urgent suspicion against the accused," the prosecutor's office said in a statement on Tuesday.
It said the suspect had made extensive statements during a police hearing, but had denied the offence.
Under German law, prosecutors have until the end of the calendar day following an arrest to seek a formal arrest warrant keeping a suspect in custody.
The prosecutor's office also said that it had been impossible to track the truck driver by witnesses after Monday's attack and that, so far, police had not been able to prove that the suspect was in the truck at the time of the attack.
It was not clear who the perpetrator was.
Thomas Maiziere, Germany's interior minister, said: "One cannot rule out that the perpetrator is still at large."
He said police had not just followed a single lead but multiple leads from the start.
It remained beyond doubt that the truck incident was an attack, Maizier said, but the motives remain unclear.
German officials, including Chancellor Angela Merkel, have called the incident a "terrorist" attack.
The truck struck the popular Christmas market outside the Kaiser Wilhelm Memorial Church late on Monday as tourists and locals enjoyed a traditional pre-Christmas evening near Berlin's Zoo station.
|German police had not just followed a single lead but multiple ones [EPA]
Dozens of people were wounded. Jan Hollitzer, a witness, told Al Jazeera the truck drove about 50 metres through the market.
Recalling the scene, Hollitzer said the vehicle was travelling "very fast".
"It was really horrible. There were many casualties and injured people," he said.
"I saw people hit by the truck and also people under the truck ... I can tell you those are images you don't want to see."
Among the dead was a man in the lorry, who died as paramedics treated him, Winfried Wenzel, Berlin police spokesman, said.
Police identified him as a Polish national, but did not give further details. The truck had a Polish licence plate, police said. The Polish owner of the truck said he feared the vehicle may have been hijacked.
Al Jazeera's Sonia Gallego, reporting from Berlin, said the atmosphere in the German capital was "mournful".
"The security officials are left with a worrying situation" following the release of the suspect, our correspondent said. "They are having to follow on ... You have an armed and dangerous person on the loose, an extreme concern for police."
She added that there were currently more tourists in the capital because of the Christmas holidays.
Source: Al Jazeera and news agencies