Berlin’s Breitscheidplatz Christmas market has reopened following the truck attack that killed 12 people and injured 49 more on December 19.

The attacker is a suspected Islamist extremist who drove the truck into a crowd.

German police have installed concrete barriers to prevent a repeat attack.

Meanwhile police have raided homes in Dortmund, but prosecutors denied reports that arrests linked to Tunisian suspect Anis Amri were made.

Anis Amri’s ID was left in the truck and now his fingerprints have been found on the door, reports say.

The truck attack victims included at least six Germans and an Israeli tourist.

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German newspaper Bild quoted the federal prosecutor’s office as saying four people who were in contact with Anis Amri had been arrested.

However, a spokesman for the prosecutor’s office denied the report, saying he was not aware of any arrests.

There were also raids at a migrant centre in Emmerich in western Germany, where Anis Amri stayed briefly last year, and at addresses in Berlin.

Anis Amri’s name came to the attention of German counter-terror services last month and he reportedly moved in the circle of extremist preacher Ahmad Abdelazziz A, known as Abu Walaa, who was arrested last month and charged with supporting ISIS.

The Ruhrnachrichten news website said Anis Amri had lived in Dortmund from time to time and residents at one block of flats said he had spent time with a German of Serbian origin, Boban S., who was arrested last month along with Abu Walaa.

Anis Amri was on a US no-fly list, had researched explosives online and had communicated with ISIS at least once via the Telegram Messenger service, the New York Times reported.

ISIS has said one of its militants carried out the attack but has offered no evidence.

Anis Amri had also offered himself for a suicide attack, Spiegel magazine reported, quoting communications intercepted for the prosecution of hate preachers in Germany.

However, what he said was not believed to be explicit enough for him to be arrested, the magazine said.

Anis Amri had also been put under surveillance in Germany earlier in the year on suspicion of planning a robbery to pay for automatic weapons for use in an attack.

However, the surveillance was reportedly called off after it turned up nothing more than drug-dealing in a Berlin park and a bar brawl.

Anis Amri, 24, is said to have entered Germany in 2015 and was due to be deported in June but stayed because there was a delay in receiving paperwork from Tunisia.

He had a history of crime, serving 4 years in an Italian prison for arson and convicted in absentia in Tunisia for a violent robbery.

A police notice lists six different aliases used by Amri, born on 22 December 1992, who at times tried to pass himself off as an Egyptian or Lebanese.

The German authorities warn the suspect could be armed and dangerous and are offering a reward of up to €100,000 ($104,000) for information leading to his arrest.

It is thought Anis Amri may have been injured in a struggle with the Polish driver of the truck, found murdered in the cab.

Investigators believe the truck was hijacked on December 19 when it was parked in an industrial zone in north-western Berlin pending delivery of its cargo.