They reveal that Anis Amri, the Tunisian suspect in the truck attack
, was part of an ISIS recruitment network that orbited around a radical preacher called Abu Walaa from al-Tamim, Iraq. The cleric, who styled himself as ISIS' representative to Germany, built up a national following among Islamist extremists in the European country.
Analysts tracking the network tell CNN as many as 20 Germans who have joined ISIS have ties to his network.
The German jihadis, like some of their compatriots, had a penchant for bureaucratic subdivision, with leading proselytizers responsible for recruiting in particular regions.
Abu Walaa was the central figure in what prosecutors called a "nationwide network of salafi-jihadi indoctrinators, which are closely interlinked and act in a work sharing manner."
The network even came up with a common curriculum to brainwash youngsters and persuade them to join ISIS. Their recruitment efforts were mainly based in the North Rhine-Westphalia and Lower Saxony regions of Germany, and they ran seminars in the towns of Dortmund, Hildesheim and Duisburg, which extremists from across the country visited. Supporters also downloaded Internet videos in which Abu Walaa's face was not featured, giving him the nickname the preacher without a face.
Just like the so-called Zerkani ISIS recruiting network in Brussels, Belgium, the German network raised funds for jihadi travel through robberies. It also raised money through applications for fraudulent loans.
Another leading figure in the Abu Walaa network was Boban Simeonovic, a 36-year-old German-Serbian from Dortmund who acted as a guru to Amri and had a reputation for fanaticism even in these radical circles.
Simeonovic stressed to followers in Germany that there was a "state of war" in the country and attacks were necessary, according to the investigative files.
Both Abu Walaa and Simeonovic were arrested in November, along with three other leading figures in the network. They were all charged with terrorism offenses.
When Amri was preparing to travel to join ISIS in late 2015, Simeonovic took him and other jihadi wannabes on long hikes with backpacks to get them in shape and tried to arrange for him to leave the country through his contacts at a mosque in Hildelsheim. It is not clear why he was not able to travel.
While in Germany, Amri communicated with the radical network using the encryption app Telegram, according to the investigative files.
Sometime before or after his travel plans fell through, Amri started discussing launching an attack in Germany, according to a police informant in the network who was cited in the investigative files. Simeonovic was in favor of this and gave the Tunisian a place to hide, the police informant told German investigators.
Other members of the Abu Walaa network discussed driving a truck full of gasoline with a bomb into a crowd, the police informant told investigators, according to the investigative files.
According to the investigative files, handwritten notes were found indicating that Simeonovic had direct contact with a number of German ISIS operatives in Syria, including the German convert Christian Emde and Silvio Koblitz, a recruit from Essen.
The investigative files also include information from Anil O., a German-Turkish defector from ISIS tied to the network, now cooperating with German authorities. The defector claimed Abu Walaa had particular clout in Raqqa, Syria, because one of his former acolytes, a German ISIS operative named Lemke, was in charge of German members of the group's Amniyat security service and was in touch with Abu Muhammad al-Adnani, who until his death was the second most powerful figure in ISIS.