German Bundesliga club Borussia Dortmund's team bus was the target of an attack in three explosions ahead of their Champions League tie against Monaco on Tuesday, police said.
"At the present moment it is still unclear what the concrete background is to this act," Dortmund police chief Gregor Lange told a news conference.
The devices exploded as the team bus left the squad's hotel some two-and-half hours ahead of the first-leg quarter-final match.
A note claiming responsibility for the attack was found near the scene and its authenticity was being "intensively examined," prosecutor Sandra Luecke said.
Dortmund's Spanish defender Marc Bartra was taken to hospital with a fracture to his right wrist and several foreign body injuries in an arm and he has undergone surgery.
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The match was postponed and rescheduled for Wednesday, ahead of two other ties including fellow German side Bayern Munich against holders Real Madrid.
The nature of the explosive devices was not immediately known, however it is believed they could have been placed in a hedge near the hotel car park.
The incident occurred in the Hoechsten district, around 10km from the club's stadium, as the Dortmund squad left the team hotel for the stadium.
After it was announced the match was being postponed, spectators already inside the ground left without incident.
Police said there were three separate explosions, bursting one of the vehicle's windows.
Dortmund's Swiss goalkeeper Roman Buerki told the Swiss newspaper Blick: "I was sitting in the back row next to Marc Bartra who was hit by shards from the burst back window.
"The bus had turned onto the main road when there was a big bang - a real explosion."
Police deployed a drone to search for other possible explosive devices in the area around the team hotel. Lange said a suspicious object was found which had the appearance of an explosive device but was not capable of being detonated.
Dortmund chief executive Hans-Joachim Watzke told Germany's Sky television there was an "explosive attack in the immediate vicinity of the hotel exit" as the bus moved from the hotel car park into Wittbraeucker Strasse.
"The team and the coaching staff are of course shocked. We must now channel it in some way," he said.
"It will not be easy to get that out of the mind. In such a crisis situation, all at Borussia pull together. I think the team will feel it tomorrow."
Club president Reinhard Rauball said: "Of course this is an extremely difficult situation for the players. But they are professionals, and I am convinced that they will put that away and will give a performance tomorrow."
Football's world governing body FIFA and European football body UEFA condemned the attacks.
