Prosecutors suspect a 32-year-old Tunisian, identified as Meher D., of having been an Islamic State (IS) instructor of the attacker, Anis Amri, and of having pushed him to carry out the attack, the daily Süddeutsche Zeitung and TV channels NDR and WDR reported based on information they acquired from Germany's Federal Court of Justice.

The suspect could be in Libya where he had joined IS in 2015, according to German justice which is looking for him for alleged membership of a terrorist group and complicity in murder, Süddeutsche Zeitung reported. 

Meher D., born in 1985, is said to have left his hometown of Tunis for Libya in 2015 in order to join the terrorist militia IS, according to German investigators. 

In the fall of 2016 D. then took over the role of looking after his Berlin-based compatriot Anis Amri from afar. 

Which rank Meher D. occupies within the IS remains unclear. His current whereabouts are also unknown, though he is still suspected to still be involved in the civil war Libya. 

Through the investigation, Germany’s Federal Criminal Police Office has worked with the authorities in Tunisia, but now are also being assisted by Germany's Federal Intelligence Service and American intelligence agencies. The Tunisian judiciary has been searching for Meher D. for some time now.

A spokeswoman for the prosecutor declined to make any comment on the reports.

On December 19, 2016, Amri, a 23-year-old Tunisian killed a dozen people and injured 48 others in the Christmas market attack claimed by IS. Amri was shot dead four days later by Italian police while on the run.