BERLIN — Anis Amri, the Tunisian man who killed 12 people in an attack on a Christmas market in Berlin in December, had been flagged weeks earlier as a high-level drug dealer, according to a newly discovered police document that has caused an uproar in Germany.

The document, which was uncovered recently during a review by the Berlin city government, has induced a new round of anguished questioning in Germany about whether the country’s worst terrorist attack in decades could have been prevented.

Berlin’s interior minister, Andreas Geisel, who announced the discovery of the document Wednesday, told city lawmakers Thursday that a thorough investigation “is what we owe the victims, their families and the survivors.’’

The document, dated Nov. 1, was uncovered by Bruno Jost, a former federal prosecutor whom the city hired to review the case.

It said that Amri was suspected of “commercial-level, gang-related narcotics trafficking’’ — charges serious enough to merit prompt police action.

Jost also found a second document, one that characterized Amri as a low-level drug offender, of a kind that would not generally warrant the most urgent police action. That document was dated Jan. 17 but was then backdated to Nov. 1, Geisel said.

At a news conference Wednesday, Geisel raised the possibility that police were trying to cover up their failure to act on the Nov. 1 document by backdating the Jan. 17 document so that it appeared that the police had conflicting intelligence.

The Nov. 1 document “would have been enough to order an arrest warrant’’ — and, potentially, jail time, Geisel said.

Geisel also said it appeared that officers with the Berlin criminal police had stopped tapping Amri’s phone in June, despite having permission to monitor it until November.

Germany’s federal interior minister, Thomas de Maizière, who was in Brussels for a meeting with counterparts from other European Union countries, said he was stunned by the revelations, which he acknowledged raised troubling questions.

“I expect all parties involved in the city-state of Berlin to investigate it very thoroughly and very openly now,’’ he said.

However, Benjamin Jendro, a spokesman for the Berlin police union, told the newspaper Der Tagesspiegel that even if the Nov. 1 warning had been acted on, “There is no guarantee that he would have been arrested.’’

Often, in drug trafficking investigations, the priority was finding the people pulling the strings, not the dealers themselves, Jendro said. “One cannot say that the colleagues have definitely made a mistake,’’ he continued, and it was impossible to say with certainty that the Christmas market attack could have been prevented.

Amri, 24, had a long criminal history that included convictions for car theft in Tunisia and arson in Italy. After moving to Germany in 2015, he improperly signed up for welfare benefits in multiple locations, and was placed under electronic monitoring.

Last spring, Amri applied for asylum, but it was denied. He was ordered deported, but he could not be quickly sent to Tunisia because he did not have a valid passport. After the attack, he made his way to the Netherlands and to Italy, where he was shot dead by police officers outside Milan.

As investigations continue, Amri’s case has become a political hot potato, both nationally and locally.

In September, legislative elections in Berlin shifted local politics left. Geisel, who is from the center-left Social Democrats, is seen by some critics as trying to score points by appearing tough on crime — and by potentially embarrassing the Christian Democrats, who shared power with the Social Democrats in the previous government.

Another Social Democrat, Ralf Jaeger, the interior minister in North Rhine-Westphalia, Germany’s most populous state, came under pressure after it became clear that the Tunisian had at the very least cheated the government by claiming welfare benefits in multiple locations.

Back then, de Maizière, an ally of Chancellor Angela Merkel’s, has accused Jaeger of failing to use his authority to detain Amri months before the attack.

Amri’s treatment played a large part in recent elections in North Rhine-Westphalia, in which the Christian Democrats argued that the state’s center-left government had been too lax on crime and the Social Democrats suffered a stunning defeat. So the new announcement by Geisel in Berlin has an element of score settling.

Merkel is seeking a fourth term in national elections scheduled for October.