German police are investigating after three explosions rocked a bus transporting the Borussia Dortmund soccer team on Tuesday night. The attack occurred at about 7:15 p.m. local time, just as the team was leaving its hotel for its Champions League match against AS Monaco. Borussia Dortmund defender Marc Bartra was wounded in the explosion and was undergoing surgery late Tuesday. “Serious explosive devices” were left near the bus on the road, police said, adding that the soccer team appeared to be the target of the attack. Investigators have not yet identified any suspects or determined a motive. “At this time, it is still not clear what the real background to this act is,” Dortmund police chief Gregor Lange said at a news conference late Tuesday. Prosecutors said earlier that they found a letter near the scene that “takes responsibility” for the explosions, but the letter’s authenticity has yet to be verified. The team’s match against Monaco has been rescheduled for Wednesday, and police say they do not believe fans due to attend the game at Signal Iduna Park stadium are in any danger.