One suspect has been detained as a note left at the scene suggests a possible Islamic extremist motive for the attack on Borussia Dortmund's team bus, German prosecutors said Wednesday.
Frauke Koehler, a spokeswoman for federal prosecutors in Germany, said investigators were focusing on two suspected Islamic extremists and have searched their homes - but authorities said a range of other motives are still possible for the three explosives that hit the bus Tuesday evening before a Champions league match. One of the suspects, a man, was arrested.
Investigators found three copies of the note at the scene. It demanded a withdrawal of Germany's Tornado reconnaissance jets that are helping to fight against the Islamic State group and a closure of the US Ramstein Air Base in Germany, Koehler said.
Federal prosecutors took over the case on the basis that a "terrorist background" is likely, but "the exact motive for the attack is still unclear," Koehler said. "An Islamic extremist background to the attack appears possible."
But Koehler said investigators were still evaluating the credibility of the note. Tobias Plate, a spokesman for Germany's Interior Ministry, said notes at the scene claiming responsibility haven't been a feature of past Islamic extremist attacks.
Three explosions went off near Dortmund's bus as the team set off Tuesday evening from its hotel on the city's outskirts for its Champions league quarterfinal match against Monaco.
They shattered a window of the bus - wounding Borussia Dortmund defender Marc Bartra, who underwent surgery for injuries to his wrist and arm. Police said an officer accompanying the bus on a motorbike suffered from blast trauma and shock. Bartra's operation went well but he won't be able to play for "several weeks," Dortmund said.
The club said the 26-year-old will watch his team's Champions league quarterfinal match on television and "is keeping his fingers crossed" for his teammates.
The devices used in the attack contained metal pins, one of which buried its way into a headrest on the bus, Koehler said. Investigators are still working to determine how the devices were detonated and what explosive substance was used.
The match was called off shortly before kickoff and rescheduled for Wednesday evening. It was being held under increased security, and the club said fans wouldn't be allowed into the stadium with backpacks.
Koehler said there are "significant doubts" about a second claim of responsibility found online suggesting a left-wing extremist motive for the bus attack.
The region's top security official raised the possibility that the note could be "an attempt to lay a false trail."
"We are investigating in every direction, and it's really meant that way," said Ralf Jaeger, the interior minister of North Rhine-Westphalia state. "It could be left-wing extremism or right-wing extremism. It could be the violent fan scene, it could be Islamic extremism."
Chancellor Angela Merkel condemned the attack as "a repugnant act" and praised the "great solidarity" shown by both teams' fans on Tuesday. She said she's hoping for a "peaceful and good game" on Wednesday night, which will be attended by German Interior Minister Thomas de Maiziere.
Borussia Dortmund president Reinhard Rauball said Wednesday's rescheduled match would be a challenge for the team.
"However, we expect and I am confident that the team will do its best and deliver a spectacle in the Champions league this evening," he said.
UEFA, European football's governing body, said security was being reviewed at all three Champions leagues games on Wednesday. It urged fans to allow extra time for tougher security.