The alleged killer in the Strasbourg Christmas market shooting had pledged allegiance ISIS, according to a judicial official, ES reported.

Cherif Chekatt, 29, is the chief suspect in the massacre which left five people dead in the French city earlier this month.

A judicial official said investigators found a video stored on a USB stick which Chekatt’s home in which he claimed allegiance to the extremist Islamic State group.

Chekatt died in a shootout with police two days after the December 11 attack and shortly after his death, the Islamic State group's Amaq news agency claimed he was a "soldier" of the group.

French Interior Minister Christophe Castaner previously rejected the claim as "totally opportunistic".

During the attack, which also left 13 others injured, witnesses said the gunman shouted "God is great!" in Arabic then sprayed gunfire from a security zone near the Christmas market.

Prior to this latest detail being made public, it was revealed Chekatt had also been on a watch list of potential extremists.