In a report to the state parliament of North Rhine-Westphalia Thursday, state interior minister Ralf Jaeger said the Joint Terror Protection Center dealt with "Anis A" at least seven times.
He was a known "troublemaker" who "traveled under at least 14 known aliases," he said. Recently Amri had been spending more time in Berlin and was being "monitored closely," though he showed no signs of planning a terrorist attack, the report found.
Amri, a 24-year-old Tunisian, was being considered for deportation at the time of the December 19 attack, but the legal requirements to deport him had not been met, Jaeger said.
At least 48 other people were injured when the tractor-trailer truck plowed through the open-air market in Berlin, which was packed with shoppers about a week before Christmas. Authorities say Amri hijacked the truck elsewhere in Berlin, killing the Polish driver, and then drove it to the center of the capital and rammed it through the market.
Amri fled after the attack and was killed in a shootout with Italian police in Milan on December 23.
Investigators have since tracked his movements to the Netherlands, France and then Italy.
A spokeswoman for the German Federal Prosecutor said last month investigators believe a video in which Amri pledges allegiance to ISIS is genuine.
Amri's path to Germany
Amri entered Italy in February 2011 without any ID and claimed to be a 17-year-old minor, Mario Viola, a spokesman for the Italian state police, said last month.
Italian authorities ordered his deportation, but Tunisian authorities wouldn't accept the request on the grounds of a lack of proper documentation, according to Viola.
At that point, Italian authorities told Amri to leave the country, and officials lost track of him.
He was not suspected of terrorism at the time and was considered a petty criminal, Viola said.
Amri was believed to have gone to Germany in July 2015 and had traveled between Berlin and other cities -- but was mostly in Berlin since February, Jaeger said in December.
He requested asylum there but his application was denied and he was then considered for deportation, officials said.