Investigators on Tuesday searched a Berlin refugee home where one 26-year-old Tunisian man lived, suspected of “having knowledge of the attack plan and possibly having helped Anis Amri” - the man who drove a truck into a crowded Christmas market on December 19th.
Amri - a 24-year-old from Tunisia - was later killed in Italy during a shootout with police.
According to investigators, the 26-year-old alleged contact man had known Amri at least since 2015 and “was still in contact with him around the time of the attack”.
Though a special police unit searched the man's home on Tuesday, investigators have not yet applied for an arrest warrant. But the man was still temporarily held due to another charge against him, according to broadcaster SWR.
Investigators also searched a Berlin apartment where a former roommate of Amri lives.
“According to investigations so far, the witness had possibly also been in contact with Anis Amri around the time of the attack,” prosecutors said, adding that the search helped to shed light on the behaviour of Amri before and after the attack.
Whether the former roommate was questioned or arrested, prosecutors did not say.
Same gun used in Berlin and Milan
Amri smuggled the weapon used in the Berlin Christmas market attack across borders to Italy and used it in the shoot-out in which he was killed, police said Wednesday.
Italian police said ballistic tests proved the gun fired at an officer in Milan was the same as the one used to kill the Polish driver of the hijacked truck.
"The weapon that killed the driver of the Berlin massacre truck is the same as the one Anis Amri used to wound a policeman in Milan," forensic police said in a statement.
An investigation was underway to see whether the weapon had been used "in other criminal episodes, in Italy or elsewhere".
Amri was shot dead after travelling from the Netherlands to France before heading to Italy.
The Tunisian was approached by two policemen as he loitered outside a Milan train station. He fired at one officer before being shot dead by the other.
The Berlin attack revealed major security flaws as it emerged that Amri had been under surveillance since March, but German police dropped their watch in September thinking he was a small-time drug dealer.
The failed asylum seeker should also have been deported months ago but Tunisia did not provide the necessary paperwork until after the attack.
Following heated criticism of the country's security measures, Interior Minister Thomas de Maizière announced on Tuesday his intention to centralize Germany's security apparatus, pulling a wide range of powers in to Berlin.
The Interior Minister is seeking greater federal powers on domestic intelligence and quicker expulsions of illegal immigrants, as well as granting federal police wider oversight across the 16 states.