Mr. Amri, who left Tunisia in 2011 and served time in six jails in Italy before arriving in Germany in July 2015, has been identified as the driver of a tractor-trailer that careened into the market on the evening of Dec. 19, killing 12 people.

Investigators have since established that he used at least eight aliases and was registered at several offices for foreigners’ affairs, mostly in Berlin and North Rhine-Westphalia, Germany’s most populous state, according to news reports.

Ms. Köhler said the death toll would have been higher, were it not for an automatic braking system that kicked in shortly after the vehicle jumped the sidewalk. Mr. Amri apparently hijacked the truck from its Polish driver at a truck lot in Berlin.

“That prevented still worse results,” she said.

Under European Union rules, heavy tractor-trailers must be equipped with an automatic braking system, which can be overridden by the driver, although Mr. Amri did not do so. The police have said the truck came to a stop about 260 feet into the Christmas market, after hitting it at an estimated speed of 35 to 40 miles an hour.