German authorities say the suspect in Monday's deadly truck attack in Berlin was under covert surveillance for several months this year.
Berlin prosecutors told The Associated Press that they launched an investigation against Anis Amri on March 14 following a tip from federal security agencies.
The tip warned that Amri, who was considered a potential threat by authorities in the western state of North Rhine-Westphalia, might be planning a break-in to finance the purchase of automatic weapons for use in an attack.
Surveillance showed that Amri was involved in drug dealing in a Berlin park and involved in a bar brawl, but no evidence to substantiate the original warning. The observation was called off in September.
A Europe-wide manhunt has been launched for the Tunisian Ami, who has been described by German authorities as "violent and armed".
Authorities issued a notice to other European countries overnight seeking the arrest of the 24-year-old, but initially held off on going public so as not to jeopardise the manhunt. After German media published photos of him and a partial name, federal prosecutors issued a public appeal for the information.
"Caution: He could be violent and armed," the notice warned. "A reward of up to 100,000 euro (£84,000) has been issued for information leading to the suspect's arrest."
A separate European arrest warrant from Germany obtained by The Associated Press states that Amri has at times used at least six different names and three different nationalities. He was described as being of average height and weight, with black hair and brown eyes.
Interior minister Thomas de Maiziere cautioned that Amri's involvement in the attack was not yet certain even though documents in his name were found in the cab of the truck
"This is a suspect, not necessarily the perpetrator," Mr de Maiziere said after briefing Parliament's domestic affairs committee. "We are still investigating in all directions."
Twelve people were killed and 48 injured when a truck ploughed into a popular Berlin market on Monday evening in an attack claimed by the Islamic State group. Twelve of the wounded were still being treated on Wednesday for very serious injuries and some were in critical condition, Berlin health officials said.
The suspect apparently arrived in Germany in July 2015 and has lived in three German regions since February, mostly in Berlin, said Ralf Jaeger, the interior minister of western North Rhine-Westphalia state.
"Security agencies exchanged information about this person in the joint counter-terrorism centre, the last time in November," he said.
State prosecutors in Berlin told The AP that they launched an investigation of Amri on March 14 following a tip from federal security agencies.
The tip warned that Amri might be planning a break-in to finance the purchase of automatic weapons for use in an attack. Surveillance showed that Amri was involved in drug dealing in a Berlin park and involved in a bar brawl, but found no evidence to substantiate the original warning.
The observation was called off in September, by which time Amri had disappeared from his regular haunts in Berlin, prosecutors said.
Separately, Amri's asylum application was rejected in July. German authorities prepared to deport him but were not able to do so because he did n ot have valid identity papers, Mr Jaeger said. In August they started trying to get him a replacement passport.
"Tunisia at first denied that this person was its citizen, and the papers were not issued for a long time," Mr Jaeger said. "They arrived today."
The claim of responsibility by IS did not identify the man seen fleeing from the truck in Berlin, but described him as "a soldier of the Islamic State" who "carried out the attack in response to calls for targeting citizens of the Crusader coalition".
Germany's top prosecutor, Peter Frank, told reporters the attack was reminiscent of a deadly rampage in the southern French city of Nice and appeared to follow instructions published by IS. He also said it was not clear whether there was one perpetrator or several in the Berlin attack.
On July 14, a Tunisian living in France was shot to death after mowing down 86 Bastille Day revellers with a truck in Nice.
Police in Berlin, meanwhile, said they had received over 500 tips on the Monday evening attack.
Christmas shoppers were out again in the streets on Wednesday in the German capital, and Berlin Mayor Michael Mueller said it was "good to see that Berliners aren't being intimidated".
"I don't think there's any need to be afraid," he told ZDF television. "The police presence has been significantly heightened ... and of course other measures taken to find the perpetrator quickly."
Mr Mueller argued there are limits to increasing security, given the number of public spaces and events.
"It wouldn't be our free and open life any more if we escalated security measures so much that people worry about going anywhere, that there are strict entry checks," he said. "We don't want that. It must be appropriate and goal-oriented."