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The Tunisian had frequently consumed cocaine and cannabis before driving a truck into a Berlin Christmas market. Italian authorities said they could not exclude the possibility he consumed drugs the day of the attack.
Italian prosecutors said while he likely had not consumed narcotics the day he was killed, they could not exclude the possibility he had been under the influence of drugs the day of the attack.
Amri last year drove a stolen lorry into a Christmas market in Berlin, leaving 12 dead and dozens more injured. He fled the scene and traveled across western Europe despite an international manhunt.
Four days after the attack, he was shot dead by Italian police on the outskirts of Milan during a routine ID check. He drew a weapon from his backpack and shot one of the Italian officers before being killed.
The "Islamic State" claimed responsibility for the Berlin attack, releasing a video in which Amri pledged allegiance to the militant group's leader Abu Bakr al-Baghdadi.
The case drew widespread criticism when Munich-based daily "Süddeutsche Zeitung" revealed Amri, a failed asylum seeker, unintentionally told a federal police informant that he wanted to "do something in Germany" and could acquire an AK-47 assault rifle in 2015.
Despite the warning, German authorities in early 2016 concluded he did not pose a threat following a months-long surveillance program, deeming him a mere errand boy. Prior to the attack, Amri had a running history of drug dealing and petty crime. German authorities had detained Amri ahead of deportation for the maximum time allowed, but he was released after Tunisian officials failed to produce identity documents for him. Germany and Tunisia on Friday agreed to an immigration deal that will speed up deportations from Germany.
In October, researchers at the International Center for the Study of Radicalization and Political Violence (ICSR) revealed that more than half of Europe's foreign fighters had a criminal record, illustrating the relationship between crime and terror.
"The presence of former criminals in terrorist groups is neither new nor unprecedented," the report said.
"But with the 'Islamic State' and the ongoing mobilization of European jihadists, the new phenomenon has become more pronounced, more visible and more relevant to the ways in which jihadists groups operate," it added.