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 ISIS.
Investigators had zeroed in on two suspects believed to belong to a large terror scene in the Ruhr region, after three identical letters were found at the scene. Only Abdul Beset A. was detained.
The letter demanded that Germany stop its Tornado reconnaissance missions in the international anti-ISIS 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 Wednesday 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.
'Informed By Text Message'
State police chief Dieter Schuermann admitted there were no strong leads.
"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 ISIS 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.
The quarter-final, first leg match was held in Dortmund just a day later in a packed stadium with tight security, with Monaco winning 3-2.
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.
'Security In Freedom'
Germany has been on high alert since a series of terrorist attacks last year, including a deadly Christmas market truck rampage in Berlin.
Chancellor Angela Merkel said Thursday that "highly varying" laws for each region under Germany's federalist system were undermining the country's fight against terrorism.
She cited differing policies on surveillance as one weakness and called for their harmonisation, in an interview with the Funke media group.
"We know that we are threatened like many other countries and will do everything in our power to ensure security in freedom for our citizens, in close consultation between the federal government and the states," she said.
Before the match began Wednesday, fans chanted "Bartra! Bartra!", in support of the injured defender.
Dortmund's chief executive Hans-Joachim Watze vowed that his side would "play not only for ourselves today. We will play for everyone... we want to show that terror and hate can never determine our actions".
Interior Minister Thomas de Maiziere, who attended the match, said the "fascination" surrounding football drove terrorists to try to disrupt it.
"That's why it's right that we do as much as we can to protect it, and not allow criminals to take the fascination away from us."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)