A 40-year-old Tunisian man has been detained in Berlin in connection with last week's attack on a Christmas market, German state prosecutors say.
They said his number was found on the phone of Anis Amri, who killed 12 people by hijacking a lorry and ramming it through the stalls.
Police raided the home and workplace of the man in the Tempelhof area of Berlin, German media report.
Officials have until Thursday to decide whether to formally arrest him.
Amri was shot dead by police in Milan early last Friday, four days on from the attack, after the 24-year-old Tunisian opened fire and injured an officer during a routine check.
Meanwhile, police in the Netherlands say it was highly likely Amri was spotted on CCTV in Nijmegen station.
They are investigating whether he travelled there immediately after the market attack, following the discovery of an unused Dutch Sim card in his backpack.
They believe he was probably handed the Sim card in Nijmegen station.
From Nijmegen, it is thought he took a six-hour bus trip to Lyon Party-Dieu station in France - from where he apparently got a train to Milan via Chambery and Turin.
The Sim card found in his backpack was issued between 20 and 22 December in one of three possible Dutch cities - Breda, Zwolle and Nijmegen, Italian media reported.
It was widely reported after the attack that German police had searched a refugee centre in Emmerich, just across the border at Nijmegen.
That Amri was able to travel from Berlin while subject to a European arrest warrant has raised security questions.