Bonsoir, the content of the letter.
Letter threatens sporting figures, celebrities
One of two claims of responsibility for the attack -- found in three copies of the same letter left at the scene -- purports to be from an ISIS sympathizer, according to a copy of the document obtained by CNN from a source knowledgeable about the investigation.

Germans open their homes to Monaco fans after Dortmund attack
The single-page letter, typed in German, calls on Germany to pull its Tornado jets from missions over Syria, calls for the closure of the US Air Force's Ramstein air base in Germany and refers to the "blessed brothers" behind the
December 2016 Berlin truck attack.
German Tornado jets have been carrying out surveillance over Syria since January 2016, regularly taking photos of potential targets for anti-ISIS coalition airstrikes.
The letter also says that famous sporting figures, actors, singers, and celebrities in Germany and "other Crusader nations" are, from now on, on the "death list of the Islamic State" unless these requirements are fulfilled.
The letter appears to have been written by a non-native German speaker or someone trying to pass themselves off as a non-native German speaker.
A second claim of responsibility, apparently from the far-left, has been circulating online, the prosecutor's office said, but there are doubts about it.
Dortmund explosion: 1 detained, terrorism suspected - CNN.com