They are also the ‘long-ball’ team of the group, making the fewest percentage of short passes (88 per cent).
But the state prosecutor claimed officials had reason to believe this letter was not authentic.
Germany's Sueddeutsche Zeitung newspaper suggested another three letters, which were found near the scene, claimed responsibility for Islamic State.
A statement from the federal public prosecutor's office claimed investigators had searched the apartments of two suspects from Germany's Islamist scene and decided to detain one of them.
Officials failed to confirm where the suspect was detained but added that a decision would be made on whether to seek a judicial arrest warrant against him.
The authenticity is currently being attested."I can't tell you more about the content because of ongoing investigations.
We are investigating in all directions."The Federal Public Prosecutor said in a statement it had taken over the investigation and would hold a news conference at midday today.It could be the secret to Borussia Dortmund's success.It's a training device which has helped the German champions run rings around City, Real and Ajax.Crucially, leaving them unbeaten in the Champions League.Jose, Roberto, Dortmund's version comprises a ball-feeding machine erected on a 14 metre square grid.We're all shocked - nobody thought of a football match in this moment."Tuesday's Champions League quarter-final first leg has now been postponed, and has been rescheduled for Wednesday, with a 5.45pm kick off.