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.
Amri got off the bus at the Lyon-Part-Dieu rail station, the source said.