The announcement marked a setback for investigators, who described the three blasts late Tuesday as a "terrorist" act and said they are focusing on suspects in the "Islamist spectrum".
"The investigation has not found evidence that the suspect took part," the prosecutors office said in a brief statement.
It said it was nevertheless seeking an arrest warrant for a 26-year-old Iraqi national, identified only as Abdul Beset A., for alleged ties to the Islamic State group.
The letter demanded that Germany stop its Tornado reconnaissance missions in the international anti-IS coalition and close the US air base at Ramstein in western Germany.
The daily Bild newspaper said police had placed Abdul Beset A. Under surveillance for several months and believed, based on tapped telephone conversations, that he might be hiding explosives in his flat.
However a raid on his home yesterday drew a blank, the report said. Even as the probe appeared to be in its preliminary stages, Dortmund officials criticised the decision to play its postponed Champions League match just 24 hours after the attack, with the perpetrator or perpetrators still at large.
"We are looking for people who are prepared to kill," he said, quoted by DPA news agency.
The head of domestic intelligence for the region, Burkhard Freyer, said investigators had not ruled out the extreme-right, the far-left or hooligans of being behind the attack.
Bild also quoted a security expert, Peter Neumann, raising doubts about an IS link to the letter found at the scene because some of its formulations were atypical for the group.
The roadside blasts left Dortmund's Spanish international Marc Bartra and a policeman injured, with the bombs "containing metal pieces" detonating minutes after the team bus set off to a planned Champions League game against Monaco.
Dortmund coach Thomas Tuchel angrily accused European football federation UEFA of treating the bomb attack as if a "beer can" had been thrown and claimed they were informed by text message that they would have to play the game a day later.
Germany has been on high alert since a series of jihadist attacks last year, including a deadly Christmas market truck rampage in Berlin.
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