Federal prosecutors said at the time that his telephone number was saved in suspect Anis Amri's cellphone and that they suspected he may have been involved.
Twelve people were killed and at least 48 more injured when attacker Anis Amri plowed through an open-air Christmas market on the evening of December 19.
Anis Amri, a Tunisian, first arrived in Europe by boat to the Italian island of Lampedusa in 2011, and was shot dead by police in Milan four days after the December 19 attack in Berlin.
In a similar attack in the French city of Nice in July, an 18-ton truck was driven for more than a mile through crowds gathered along a main street for Bastille Day celebrations, killing 84 people and injuring many more.
The investigations so far have "resulted in no connections of Anis Amri to possible accomplices or supporters in Milan and in the Milan region", said Antonio de Iesu.
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.
Koehler confirmed media reports that the 40-tonne truck came to a rest after 70-80 metres (230-260 feet) thanks to its automatic braking system that activates when impacts are detected.
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A black box-style system which records truck movements, speed, and driver activity is also required.
Surveillance cameras filmed Amri at the station last Thursday.
Franco Roberti, Italy's head of anti-terrorism, said Amri had been radicalised since arriving in Europe.
Police also found a Dutch Sim card inside the backpack of Amri after his death, which may have been handed over to him by an unidentified person in a train station in Nijmegen, The Netherlands.
Italian investigators are trying to determine whether Amri was tapping a jihadi network in Italy, his port of entry to Europe in early 2011 amid the Arab Spring upheaval.
But so far, said Prime Minister Paolo Gentiloni, "the investigation has not revealed any particular networks that Amri would have had in Italy".
Authorities in Rome, meanwhile, seized cellphones during a search of two residences in Rome where Amri stayed in 2015, Italian news agency ANSA reported.