Amri, 24, went on the run and was the focus of a four-day manhunt before being shot dead by police in Milan, northern Italy, after opening fire first.
German police said they found his fingerprints and his temporary residence permit in the cab of the truck used in the Berlin attack, next to the body of its registered Polish driver, who was killed with a gunshot to the head.
The autopsy on Amri's body has still not been completed, officials told dpa.
The Berlin rampage was claimed by Daesh, which released a video last Friday in which Amri is shown pledging allegiance to Daesh chief Abu Bakr al-Baghdadi.
By bus and train
More than a week after the attack, investigators were still battling to find out if Amri had help before and after the assault.
Three other men, including Amri's nephew, were arrested by Tunisian authorities last Friday.
On Wednesday, a spokesman at the anti-terrorism unit told AFP that their probe was ongoing, declining to give further details.
Separately, investigators came closer to tracing Amri's escape route to Milan.
The Tunisian had boarded an overnight bus at the Dutch city of Nijmegen, near the German border, that took him to Lyon in central France, sources close to the investigation said.
Wim de Bruin, spokesman for the Dutch public prosecution service told AFP: "We believe he was in Nijmegen, most likely last Wednesday."
De Bruin said there were "signs" 24-year-old Amri had passed though the Netherlands on his way to France.
"There are video images and it's very likely him," De Bruin said, adding that "it's most likely here where he received a SIM card," which Italian police later found on his body.
Amri got off the bus at the Lyon-Part-Dieu rail station, one of the sources 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.
A train ticket from Lyon to Milan via Turin was also found on Amri's body.
But investigators are still trying to determine how Amri was able to leave Berlin and cross most of Germany to reach the Netherlands.
Amri was known to Tunisian police as a juvenile delinquent who drank and took drugs.
In 2011, he left his home country for Italy. There he spent four years in prison for starting a fire in a refugee centre, during which time he was apparently radicalised.
After serving his sentence he made his way to Germany in 2015, taking advantage of Europe's Schengen system of open borders -- as he did on his return to Italy last week.
German security agencies began monitoring Amri in March, suspecting he was planning break-ins to raise cash for automatic weapons to carry out an attack.
But the surveillance was halted in September because Amri, who was supposed to have been deported months earlier, was seen primarily as a small-time drug dealer.
