BERLIN—A 20-year-old German man went on trial in Frankfurt on Monday on charges he was a member of the extremist Islamic State group, the first such case to come to court in Germany.

Prosecutors allege that Kreshnik Berisha, who once played for a Jewish football club, travelled last year to Syria and fought with the group before returning to Germany five months later.

Berisha, who faces up to 10 years in prison for membership of a foreign terrorist organization, was arrested at Frankfurt airport in December and has been held in detention since.

Heightened security measures have been put in place for the trial, which is expected to last until at least November. It opens just days after Germany formally banned Islamic State symbols and any propaganda activity for the group.

Authorities say more than 400 people from Germany have joined jihadist groups fighting in Syria since the start of the conflict there. Some are converts to Islam, but many — like Berisha — come from Muslim families that have settled in Germany.

Berisha’s radicalization apparently occurred over the course of a few years. As recently as 2011 he appeared “very nice, unassuming,” according to the chairman of TuS Makkabi Frankfurt, Germany’s most prominent Jewish football club, whose under-17 youth team Berisha played for.

“There were no signs of what would follow later,” said Alon Meyer, who coached the youth team one level above the one Berisha played on.

“This shows just how strong the forces must have been that turned a well-integrated player into an aspiring terrorist,” he told The Associated Press in a telephone interview.

Meyer said the speed with which Berisha went from playing for a Jewish football club to joining a radical jihadist group underlined “how strongly we have to confront this kind of racism or we are in real trouble.”

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