When Tunisian terrorist Anis Amri drove a truck into a crowd of people on Breitscheidplatz, killing 11 people, Germany's domestic intelligence and police were already well aware of who he was.
He had first come to the attention of anti-terror authorities in North Rhine-Westphalia (NRW), when he took part in a training session for potential Isis recruits a year before his deadly attack.
Since then he repeatedly appeared on the radar of authorities in NRW and Berlin.
But a legal system which requires a high burden of proof and a Tunisian state unwilling to take back a known extremist prevented him from being caught, two newspaper reports from Thursday claim.
The Abu Walaa connection
When Amri took part in a 16-kilometre walk through the west German countryside on December 23rd 2015, carrying a heavy backpack on his back, he was doing so under the supervision of Abu Walaa, the Kölner Stadt Anzeiger (KSA) reported on Thursday.
Real name Abdulaziz Abdullah Abdullah, Abu Walaa is considered one of the most influential and dangerous Islamist preachers in Germany.
In November 2016, he was arrested along with four associates on suspicion of supplying fighters to Isis in Syria.
But according to the KSA's security sources, Abu Walaa and his men had been preparing attacks in Germany, too.
During investigations into the group, police managed to smuggle two informants into their network, who reported that by autumn 2015, they had already bought guns and hand grenades in preparation for an attack on a police station.
Another source reported that the extremists wanted to fill a truck with petrol and explosives before driving it into a crowd of people.
Nonetheless, police never gathered enough proof of concrete attack plans to satisfy Germany's judiciary that the group should be arrested.
Calls to attack
It was in the Abu Walaa milieu that Amri became radicalized, first intending to travel to Syria to wage jihad for Isis, according to the KSA.
But by early 2016, investigators had become aware that the 24-year-old was calling on other extremists to join him in an attack on German soil.
Amri looked up on the internet how to build bombs, he tried to get hold of automatic weapons, and according to one undercover agent, he boasted that he would carry out a blood bath.
But again, a lack of concrete evidence that he had taken steps to carry out such an attack meant that police could not arrest him, according to the KSA.
In July 2016, the high burden of proof required by German law seemed to again prevent authorities from taking Amri off the streets.
Paragraph 58a of the deportation law allows for a “special risk” person to be deported at speed. But intelligence services decided that they didn't have a case against Amri that would convince a judge.
The nail in the coffin for investigators came in September.
Berlin police had been tapping Amri's phone since he moved to the capital in March. But when they went to a judge to ask for an extension, the only evidence they had that would stand up in court was that he was involved in drug dealing.
German law allows for drug dealers to be tapped for a maximum period of six months, so the wire tap was stopped.
Tunisia stalls deportation
Without enough proof to have Amri detained under paragraph 58a of German deportation law, the best chance to make him leave was to have Tunisia send over the paperwork proving he was a Tunisian citizen.
This should have been straightforward enough. Amri's asylum application had been rejected in the early summer of 2016, but because he wasn't carrying his real Tunisian ID, he couldn't be sent back to his homeland.
But Tunis failed to respond to repeated requests from Berlin for a quick resolution of the situation, despite Germany emphasizing in July how urgent the matter was, the Süddeustche Zeitung reported on Thursday.
In October the Tunisians finally gave their response.
There had been a mistake: Amri was not a citizen of their country, they told their German counterparts.
It was a reply Germany had heard before. Tunisia has been hit repeatedly by bloody terror attacks in recent years and is reluctant to take back problem cases like Amri.
Three days later, Germany made a new request for papers, this time using a fingerprint from Interpol to prove that Amri was in fact a Tunisian citizen.
On December 21st, Tunisia finally delivered the official paperwork, two days after Amri had killed 12 people in west Berlin.