A Europe-wide manhunt is under way for the Tunisian man wanted for the Berlin lorry attack, who had been under surveillance earlier this year, media reports say.
Anis Amri, 24, was reportedly monitored on suspicion of planning a robbery in order to pay for guns but surveillance was lifted for lack of evidence.
Before entering Germany, he had served four years for arson in Italy.
Monday evening's attack at a Christmas market killed 12 and injured 49 more.
An arrest warrant was issued after Anis Amri's residence permit was found in the cab of the lorry that left a trail of carnage at a Christmas market near Berlin's most famous shopping street, the Kurfuerstendamm.
The German authorities warn he could be armed and dangerous and are offering a reward of up to €100,000 (£84,000; $104,000) for information leading to his arrest.