According to Die Welt's currently unconfirmed information the arrested perpetrator is a Pakistani and not, as previously reported, a Chechen....
He is said to be known to the police for petty crimes, but not a terrorist background. This is reported by the Berlin "Tagesspiegel" citing sources in the security services.Source
Die Welt also reports he came to Germany as a refugee. Thanks Merkel.
The suspect, whom Berlin police arrested in the night, is said to be a Pakistani or an Afghan. This is being reported by the Deutsche Presse-Agentur [German Press Agency] citing security sources. In addition, according to information from the security authorities, he may have arrived in Germany as a refugee in February 2016.
There is still no absolute certainty initially as the man has used different names and thus it has not been possible to clarify his identity beyond doubt. Many things point to the man - a Pakistani or Afghan - having registered as a refugee when he arrived in Germany.Source
The Paki used the names Navid B. and Naved B and was staying at refugee accommodation on the grounds of Berlin's abandoned Tempelhof airport.
UPDATE: Die Welt reports high-ranking Berlin police officers saying this Paki was not the perpetrator of the attack; in other words, they have the wrong person.