BERLIN - German police are in a race against time to find a 24-year-old Tunisian considered to be dangerous and possibly armed and who is wanted in connection with Monday’s deadly truck attack on a Berlin Christmas market.

The authorities on Wednesday offered a reward of up to 100,000 euros ($104,000) for help in finding Anis Amri, who was already under surveillance in Germany for possibly planning a serious act of violence and is now the subject of a search across Europe.

Amri, who has several aliases and was facing deportation, is believed to have fled the scene of Monday’s attack, which left 48 people injured and 12 people dead, including the truck’s Polish driver, who was found shot, stabbed and bashed in the cabin of the truck.

German Interior Minister Thomas de Maiziere said the gun involved in the shooting is missing.

Identity documents belonging to Amri were found in the truck after the attack, which has left the nation’s capital badly shaken.

Fourteen of those injured in the carnage remain in life-threatening condition.

On Wednesday, the authorities stepped up their efforts to find Amri by issuing an official photo of him. They described him as 5 feet 10 inches tall, weighing about 165 pounds, with black hair and brown eyes.

As the search for Amri picked up speed, Christmas markets across Berlin began to reopen on Wednesday after a day spent shuttered after the truck attack near one of Berlin’s main shopping streets.

Mourners have also been leaving flowers, candles and messages of condolences at the scene of the attack at Breitscheidplatz surrounding the city’s landmark Kaiser Wilhelm Memorial Church.

Berlin Mayor Michael Mueller praised the city’s residents in a TV interview, saying it was “good to see that Berliners aren’t intimidated” and that the city was “standing together for our liberal life in Berlin.”

However, an anti-foreigner group held a rally outside Chancellor Angela Merkel’s office to protest her handling of Germany refugee policy on Wednesday evening.

At the same time, a left-wing group rallied near the attack site to commemorate the victims and to protest against those who they said were sowing the seeds of hate.

The Interior Minister in the western German state of North Rhine Westphalia, Ralf Jaeger, said Amri’s asylum request had been rejected in June. However, the authorities lacked the necessary documents from Tunisia to deport him.

Jaeger said Amri, whom Tunisian radio station Mosaique said was born in the province of Kairouan in 1992, had been living in Berlin since February, but had also stayed in North Rhine Westphalia. He came to Germany in July last year.

Germany’s domestic intelligence agency considers Amri to be a so-called Gefaehrder (danger to others) who is prepared to commit acts of terrorism at any time.

The intelligence services are currently monitoring 549 of these potential terrorists and are being helped in the Amri investigation by the Tunisian Foreign Ministry.

The German Bild tabloid also linked Amri to Ahmad Abdelazziz A - an Iraqi man widely known under his nom de guerre Abu Walaa - and who is the suspected head of a group that recruited for and provided financial and logistical support to the Islamic State terrorist group in Germany.

Ahmad Abdelazziz A was arrested in western Germany alongside four other suspected extremists in November.

The Islamic State claimed responsibility for the attack through the Amaq news agency, the group’s semi-official mouthpiece, saying that one of its soldiers had acted in response to calls to target citizens of the coalition of states fighting the terrorist group.

The Islamic State often issued claims of responsibility for attacks in which it had no role, or only a limited one.