Militant group ISIS are claiming they inspired the deadly Berlin market attack which left at least 12 people dead.
German Interior Minister Thomas de Maiziere says investigators are following several leads, with those responsible still being investigated.
Islamic State's claims came via a United States AMAQ news station.
Police are still hunting for the driver of the truck in the deadly attack, who is still at large.
Authorities detained a Pakistani refugee on Tuesday (local time) on suspicion of being the driver, but he has since been released from custody, German authorities say.
A Polish citizen was found dead in the truck, however he was not the driver.
"The investigation up to now did not yield any urgent suspicion against the accused," the prosecutor's office said in a statement on Tuesday (local time).
The statement said the suspect had made extensive statements during a police hearing, but had denied the offence.
It added it had been impossible to track the truck driver by eye-witnesses following the attack and that the investigation so far had not been able to prove that the suspect was in the truck's cab at the time of the attack.
Doubts were raised after the arrest over whether police had the right suspect.
"It is in fact unclear whether (the suspect) was the driver," a police chief said during a press conference on Tuesday.
Berlin police tweeted Tuesday afternoon (local time) that "the temporary arrested suspect denies the offense. Therefore we are particulary alert. Please also be alert".
On Monday night the truck smashed into wooden huts serving mulled wine and sausages at the foot of the Kaiser Wilhelm memorial church, one of West Berlin's most famous landmarks.
Twelve people were killed, while 48 were injured - 18 severely.
New Zealand journalist Cathrin Schaer lives in Berlin said people around the city are on high alert.
"You're looking around, [asking] 'is that guy acting suspicious?', 'what's that person doing?'. That's the feeling on the street," she told Newshub.
Ms Schaer said although people have been horrified by the attack, it hasn't come as a surprise to many locals.
"I think people in Berlin were expecting this actually. I think people here know what happened in other places… and I think a lot of people here have been waiting for this to happen, so I don't think it's a surprise.
"You usually feel quite safe in Europe, but I think that's changed over the past year for sure."
United States President Barack Obama spoke with German Chancellor Angela Merkel to offer his condolences and the help of the US after the attack.
"The President reiterated the US offer of assistance and underscored that no attack could sway our determination - and that of our German allies - to defeat terrorism in all of its forms," a statement from the White House said.