He was arrested, but in a separate case.
The 26-year-old Tunisian had known Amri since the end of 2015 and the pair ate together at a Berlin restaurant the night before the December 19 attack, said Frauke Koehler, a spokeswoman for federal prosecutors. His quarters at a refugee home were searched yesterday.
The pair's meeting led prosecutors to believe that the man may have been involved in the attack or at least knew that Amri planned to commit one, Koehler said.
The man was, however, detained yesterday in a separate case run by Berlin local prosecutors, Koehler said. Berlin prosecutors said the arrest was for allegedly falsely claiming benefits.
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A further search was conducted yesterday at the home of a one-time roommate of Amri who is being treated as a witness in the case, Koehler said.
Amri, a 24-year-old Tunisian, tried to reach the former roommate on the morning and afternoon of December 19, but it isn't clear whether they actually spoke, she added.
Koehler said that the Polish driver apparently was fatally shot before the truck set off for the market from its parking place north of central Berlin. She said investigations have shown "no indications that there was a third person in the cab at the time of the attack."
Germany on December 21 released a Europe-wide wanted notice for Amri, who used a string of different names and nationalities. He was killed in a shootout December 23 with Italian police in a Milan suburb after they stopped him for a routine identity check.
The pictures "suggest that Amri was aware that he was being recorded by this video camera," she said. She added that he turned to the camera and showed a "tawhid finger", or a raised index finger -- an Islamic gesture sometimes used by jihadis.
The Islamic State group claimed responsibility for the attack days afterward.
On December 21, footage from cameras in the Netherlands suggests that he was at the railway station in Nijmegen and about two hours later at the station in Amsterdam, Koehler said.
Italian investigations say that the weapon Amri used to shoot at a policeman in Milan is identical to the one used on the Polish truck driver in Berlin, Koehler said.
Investigators are still trying to figure out how he got hold of it, but that's difficult because manufacturer Erma went bankrupt at the end of the 1990s, she added.