Paris (CNN) The man responsible for a fatal Christmas market attack in the eastern French city of Strasbourg pledged allegiance to ISIS, a judicial source with knowledge of the investigation told CNN.

"A video in which Chérif Chekatt pledged allegiance to ISIS was found on a USB key," the source said. It was found during the police search, the source said, but declined to provide further information.

Cherif Chekatt was killed in a shoot-out with police.

Following the gun and knife attack, ISIS claimed through its Amaq news agency that the assailant was a "soldier" of ISIS. However, the terror organization offered no evidence of advanced knowledge or planning involved in the attack.

Chekatt, 29, was known to prison officials for being radicalized and for his proselytizing behavior in detention in 2015, Paris prosecutor Rémy Heitz said earlier this week, adding that he had been incarcerated multiple times in the past.

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