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 say 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 government officials have said the lorry’s automatic braking system may have prevented the loss of more lives, German media reported.
The system kicks in when it senses an impact and this may have cut the hijacker’s rampage short.
It was initially believed that the lorry’s original driver, Lukasz Urban, had intervened, possibly grabbing the steering wheel, while being held hostage in the passenger seat.
But doctors later said Urban would have been unconscious by this point, having been shot some hours before when Amri stole the vehicle from him.
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-Part-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.