There are "significant doubts" that the attack on the Borussia Dortmund team bus ahead of Champions League tie was the work of Islamic extremists, it has been reported.
An investigation by two German broadcasters NDR and WDR and the Suedeutsche Zeitung newspaper suggested that the three letters found on the scene were planted in an attempt to lead people to believe Islamists were involved.
According to the Suedeutsche Zeitung experts believe that the letters were written by a German speaker who included deliberate mistakes to suggest that the author was a foreigner.
The letters included demands for the closure of the Ramstein air force base and the withdrawal of German planes from Syria.
It is not usual practice for the Islamic State to make demands or negotiate in this manner, Frauke Koehler, the state prosecutor said.
He also confirmed that he regarded the media reports as accurate.
One theory is that the devices were planted by somebody from the far right or football hooligans.
Another newspaper, Tagesspiegel, said it had received an email which made racist threats and described the bombing as a warning.
Two people, including Dortmund's Spanish defender, Marc Bartra, needed medical treatment.
Bartra underwent wrist surgery following the bomb and is not expected to play for several weeks.