"The deceased suspect Anis Amri had saved the number of this 40-year-old Tunisian national in his phone. The investigations indicate that he could have been involved in the attack," the federal prosecutor's office said in a statement.
The suspect was taken into custody early today after federal police officers searched his home and work premises.
"The extent to which the suspicions against the detained person can be confirmed remains subject to further investigation," the statement added.
The 24-year-old went on the run and was the focus of a frantic four-day manhunt before being shot dead by police in Milan after opening fire first.
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The Berlin rampage was claimed by the Islamic State group, which released a video last Friday in which Amri is shown pledging allegiance to IS chief Abu Bakr al-Baghdadi.
Tunisian authorities on Saturday announced that they had arrested Amri's nephew and two other men suspected of being members of a "terrorist cell" connected to Amri.
Meanwhile, sources close to the investigation today said Amri travelled by bus from the Netherlands to France before heading to Italy where police shot him dead.
Two days after the attack left 12 dead, 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.
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.
Wim de Bruin, spokesman for the Dutch public prosecution service told AFP: "I can confirm that the Dutch police are investigating whether he travelled through The Netherlands after the attack in Berlin."
De Bruin declined however to give further details.