German federal prosecutors investigating the three explosions that hit the Borussia Dortmund team bus on Tuesday night said they had detained a suspect.
At a news conference on Wednesday, spokesperson Frauke Kohler said a letter found near the scene suggested a possible Islamist extremist motive.
She said two suspects from the "Islamist spectrum" were currently at the centre of the investigation. Their flats were searched and one person was detained.
The Office of the Attorney General of Germany is investigating after the explosions rocked the Dortmund bus as it drove to their home stadium for a Champions League match against Monaco, injuring defender Marc Bartra and causing the game to be postponed.
The letter found at the scene demands the withdrawal of German Tornado reconnaissance jets from Turkey and the closure of the United States' Ramstein Air Base in Germany.
Kohler said officials were still evaluating its credibility, adding that they had "significant doubts" about an online claim of a left-wing extremist motive.
"It is to be assumed that there is a terror background," a Federal Prosecutor's office statement said. "The motives are yet unclear.
"There were three claims of responsibility near the scene of the attack. They all had the same content, and an Islamist background can't be ruled out at this point.
"Two suspects from the Islamist scene have been in the focus of our investigations. We raided their flats, and one has been detained temporarily.
"It is currently being considered to issue an arrest warrant."
Tobias Plate, a spokesman for Germany's interior ministry, said notes claiming responsibility at the scene had not been a feature of previous Islamist attacks.
The Federal Prosecutor's office statement said the explosive devices used "were armed with metal pins, one of them drilling itself into one of the headrests of a seat. We can be thankful that nothing more serious happened."
It said the explosive power was "up to 100 metres."
Speaking shortly before Kohler, Ralf Jager, the region's leading security official, raised the possibility that the note found at the scene could be "an attempt to lay a false trail."
"We are investigating in every direction," Jager, the interior minister of North Rhine-Westphalia state, said.
"It could be left-wing extremism or right-wing extremism. It could be the violent fan scene, it could be Islamist extremism."
Steffen Seibert, a spokesman for German Chancellor Angela Merkel, called the explosions "a repugnant act" and said Merkel had spoken to club officials.
Seibert added that the chancellor was "dismayed" by the attack.
Dortmund confirmed Bartra had suffered a broken wrist and will be out several weeks, while police in the city said an officer who was escorting the bus was also injured.
Club president Reinhard Rauball said Wednesday's rescheduled match would be a challenge for the players but added: "I am confident that the team will do its best and deliver a spectacle in the Champions League this evening."
German police have put extra security measures in place ahead of Real Madrid's Champions League quarterfinal at Bayern Munich.
UEFA said security at all three of Wednesday's Champions League games was being reviewed.
Information from The Associated Press was used in this report.