Mr. de Bruin said that the police were following up on reports from people who thought they might have seen Mr. Amri. “We have to investigate the reports because we would like to reconstruct the route he took in the Netherlands to get from Germany to France,” he said in the interview.

He added that investigators were working closely with their counterparts in Germany, France and Italy.

Mr. Amri is said to have careened into a Christmas market at the symbolic Kaiser Wilhelm Memorial Church in Berlin just after 8 p.m. on Dec. 19. The police initially detained a Pakistani man who was found to have no connection to the truck and thus the assault. The error ended up giving the suspect a head start of almost 20 hours to flee, before investigators scouring the cab found a migration document that led to Mr. Amri.

Mr. Amri, who had a history of petty crime and used several aliases in his odyssey around Europe, applied for asylum in Germany in April. His application was rejected in June, and he was ordered deported, but he managed to slip through the cracks.

He may have benefited from Germany’s decentralized political system. Power is spread over 16 states, and police, judicial and migration officials have distinct spheres of authority.

For example, Mr. Amri was detained for two days in the southern German town of Friedrichshafen on July 30, after trying to take a bus to Zurich, when the police noticed he was under deportation order. But an office for registering foreigners in Kleve, in the far northwest of Germany, which was responsible for the order, said it did not have the papers from Tunisia necessary to carry out the deportation, so Mr. Amri was ordered released.