Federal investigators on Wednesday detained a 40-year-old Tunisian man in Berlin believed to have helped Anis Amri, the man who attacked a Christmas market, killing 12 people.
Federal investigators (BKA) searched two properties belonging to the man, an apartment and a business in the Tempelhof district in the south of the capital.
"The deceased suspect Anis Amri had saved the number of this 40-year-old Tunisian national in his phone. The investigations indicate that he may have been linked to the attack," federal prosecutor's office said in a statement.
Police began investigating the man after looking into telecommunications data they had secured on Amri, Spiegel reports.
In the cabin of the truck investigators found the 24-year-old's mobile phone as well as his ID papers.
Federal prosecutors are now considering whether to seek a longer arrest warrant for the man.
Federal investigators announced over Christmas that they were focusing on whether Amri had help from accomplices, after the young Tunisian was shot dead in Milan on Friday.
"It is very important for us to determine whether there was a network of accomplices... in the preparation or the execution of the attack, or the flight of the suspect," federal prosecutor Peter Frank told reporters.
Amri took over a haulage truck before driving it into a crowded Christmas market in western Berlin on December 19th.
Twelve people died in the attack and dozens were injured which has since been claimed by terror group Isis.
Last week Isis released a video in which Amri swore allegiance to the group.
Stop over in Netherlands
Amri travelled by bus from the Netherlands to France before heading to Italy where police shot him dead, sources close to the investigation said Wednesday.
Two days after the December 19 attack, the 24-year-old Tunisian boarded an overnight bus at the Dutch city of Nijmegen, near the German border, that took him to Lyon in central France, one of the sources said, confirming a French media report.
Amri got off the bus at the Lyon-Part-Dieu rail station, the source said.
Surveillance cameras filmed Amri at the station last Thursday.
From there, he took a train to the French Alpine town of Chambery before heading to Milan, in northern Italy.
Italian police shot Amri dead in the early hours of Friday after he fired at officers who had stopped him for a routine identity check.
A train ticket from Lyon to Milan via Turin was found on his body.
Investigators are still trying to determine how Amri was able to leave Berlin and traverse most of Germany to reach the Netherlands.