Investigators reportedly have "significant doubts" about the authenticity of letters claiming responsibility for this week's bomb attack on German football club Borussia Dortmund's bus.

Three still-unverified confession letters found at the scene of Tuesday evening's blasts had suggested an Islamic extremist motive.

However a report commissioned by investigators - reported by German broadcasters NDR, WDR and newspaper Sueddeutsche - said detectives now question whether the letter was really written by Islamists.

German prosecutors confirmed on Thursday that a man detained in connection with the bus attack is believed to be a former commando unit leader for the so-called Islamic State in Iraq.