(CNN) Investigators have concluded that a video in which the suspected Berlin Christmas market attacker pledges allegiance to ISIS is genuine, a spokeswoman for the German Federal Prosecutor said Thursday.

In the video -- released last Friday on ISIS-affiliated website Amaq -- Anis Amri pledges allegiance to the terror group's leader, Abu Bakr al-Baghdadi. He vows that "we will slaughter" the "crusaders who are shelling the Muslims every day."

Speaking at a press conference in the city of Karlsruhe, Frauke Koehler said: "The video claiming responsibility ... is, according to our investigation, authentic. It shows Anis Amri."

Twelve people were killed and at least 48 more injured when a truck plowed through an open-air Christmas market in Berlin on the evening of December 19.

Amri, a 24-year-old Tunisian and the suspected driver, then fled Berlin and was killed in a shootout with Italian police in Milan on December 23.

Koehler said investigators have since tracked Amri's movements to the Netherlands, France and then Italy.

Tunisian man freed

Koehler also said that a 40-year-old Tunisian man detained earlier this week on suspicion of being linked to Amri has now been released.

Investigators had thought the Tunisian man was the recipient of a voice message and picture sent by Amri, she said.

"Further investigations have shown, however, that the arrested person was not the possible contact person of Anis Amri. This is why he had to be released," Koehler said.

Investigators are working flat out to trace Amri's possible links to a broader network, Koehler said.

German media outlets NDR, WDR and Sueddeutsche Zeitung reported Wednesday that Amri had sent a picture of himself from the truck prior to the attack, with the message: "My brothers everything is ok, so God wishes. I am in the car, praying for my brother, praying for me."

Braking system 'saved lives'

Authorities believe an automatic braking system installed in the truck saved lives by halting its deadly path through the crowded market, Koehler said.

This was the reason that "worse consequences were prevented," said Koehler.

The automatic braking system sensed an impact and applied the brakes, reports by German media outlets NDR, WDR and Sueddeutsche Zeitung said Wednesday.

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A spokesman for the truck manufacturer, Scania, confirmed to CNN that the vehicle used in the attack -- a Scania R450 semi-trailer -- was fitted with an automatic braking system as required under EU regulations passed in 2012 for all trucks above a certain size. A black box-style system which records truck movements, speed, and driver activity is also required.

Scania spokesman Hans-Åke Danielsson said the company had provided police with that information.

According to Scania, it is possible for the driver to override the automatic braking system.

In a similar attack in the French city of Nice in July, an 18-ton truck was driven for more than a mile through crowds gathered along a main street for Bastille Day celebrations, killing 84 people and injuring many more.

Ballistics examination

A postmortem examination of a Polish driver whose truck was hijacked and used in the Berlin attack showed he had been shot dead before his vehicle plowed into the market, Koehler said.

Full details from the autopsy won't be released until January, she said, adding that there was no sign of stab wounds on the body.

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A .22 caliber weapon was used in both the Berlin and Milan incidents but it is not yet known if it was the same gun, Koehler said. Ballistics specialists are working to establish that, she said.