Police in Germany are investigating a far-Right claim of responsibility for last week’s bomb attack on a Borussia Dortmund team bus, amid growing doubt over whether it was the work of Islamic State.

A anonymous email sent to Tagesspiegel newspaper on Friday claimed responsibility on behalf of a far-Right group and threatened a further attack in Cologne next weekend.

Police are taking the new claim seriously but there are believed to be doubts over the authenticity of the email.

Marc Bartra, a centre-back for Borussia Dortmund, suffered injuries to his hand when remote control bombs were set off as the team bus was leaving a hotel on Tuesday.

The new email, which praises Adolf Hitler and rails against mulitculturalism, claims the Dortmund attack was a “final warning”.

It threatens an attack on protestors who plan to demonstrate against the far-Right Alternative for Germany (AfD) party’s conference in Cologne next Saturday.