DORTMUND, Germany (AP) — German police said Wednesday they are investigating "in all directions" after three explosions went off near Borussia Dortmund's team bus ahead of a Champions League quarterfinal match, injuring one of the soccer team's players.

German news media reports citing unnamed sources said a note found at the scene contained radical Islamic rhetoric but that police were unsure whether that was genuine or an attempt to throw them off the track. Federal prosecutors took over the investigation; they typically handle especially serious cases, including those in which a terrorist motive is suspected.

The first-leg match against Monaco was called off shortly before kickoff Tuesday evening following the blasts near the team hotel in suburban Dortmund, which authorities assume were a targeted attack. Players and police were preparing to go ahead with the rescheduled match on Wednesday evening, with heavy security in place.

Investigators were checking the authenticity of a letter claiming responsibility that they found near the scene, and were refusing to give any details of its contents, citing the ongoing probe. Police spokeswoman Nina Vogt told ZDF television that "of course our investigations have to go in all directions to begin with."

She said the direction of the probe remains "open."

Die Welt newspaper and Sueddeutsche Zeitung reported online that the note made reference to Germany's participation in the military coalition opposed to Islamic State. German Tornado reconnaissance planes are taking part in a noncombat role. The papers said that experts on Islam were reviewing the note to try to determine its authenticity.

Meanwhile the dpa news agency reported that investigators were probing statements found on the web that indicated a left-wing motive to punish the club for allegedly not opposing racism strongly enough.

Federal prosecutors have scheduled a statement for 1200 GMT.

Spanish defender Marc Bartra suffered injuries in the wrist and arm and underwent an operation Tuesday night. Police said an officer who was accompanying the bus on a motorbike was suffering from blast trauma and shock.

Borussia Dortmund president Reinhard Rauball said he was confident that the team would rise to the occasion for Wednesday's match.

"How is the team doing? You can imagine yourself whether they slept well or badly," he said. The players were to gather Wednesday morning to digest Tuesday's events and prepare to face Monaco.

"This is certainly a difficult situation for the team which, I think, has not experienced such a situation before," Rauball said. "However, we expect and I am confident that the team will do its best and deliver a spectacle in the Champions League this evening."